Title: Means and methods for detecting, producing, isolating or characterizing influenza The invention relates to the fields of biology, medicine and immunology. Influenza is an infectious disease that can be caused by four types of Influenza viruses; types A, B, C and D. Types A, B and C are known to affect humans. To date there are no reported cases of human infections by Influenza type D. Influenza viruses, belonging to the family of Orthomyxoviridae, are negative- sense, single stranded RNA viruses consisting of eight negative single-stranded RNA-segments (influenza A and B), or seven negative single-stranded RNA- segments (influenza C). Influenza viruses infect millions of people every year. Symptoms of influenza include symptoms that are comparable with the common cold, such as coughing, sneezing, runny nose, headache, muscle and joint pains, fever, chills and sore throat. However, influenza can also lead to life-threatening complications such as pneumonia and lead to death in high-risk groups such as young children, the elderly and immunocompromised individuals such as for instance chronically ill individuals or transplant recipients. Influenza virus particles (virions) are about 80-120 nanometers in diameter. The core of a virion contains the viral genome and proteins for packaging and protection of the viral RNA. Two proteins are present at the viral surface: hemagglutinin (HA) and neuraminidase (NA). The HA protein mediates binding of the virion to target cells, followed by cell entry. HA consists of two subunits, HA1 and HA2, linked by disulphide bonds. The major part of HA1 forms the globular head region of HA, while HA2 mainly forms the stem region of HA. Sixteen HA serotypes are currently known, which differ mainly in their globular head region. The HA stem region is more conserved. HA must be cleaved by host proteases to yield the two polypeptides HA1 and HA2 in order to be infectious. Following cleavage, the exposed N-terminus of the HA2 polypeptide acts to mediate fusion of the viral membrane with the host cell membrane, allowing the virus to infect the host cell. The neuraminidase (NA) protein is needed for the release of new virions from infected cells. NA catalyzes the hydrolysis of terminal sialic acid residues of glycoproteins of the host cell, thereby preventing binding of HA to these proteins. NA thus facilitates release of the virus from a cell and consequently spreading of the virus. Influenza viruses can be subdivided into different types based on envelope protein expression. Currently 18 hemagglutinin serotypes (H1-H18) and 11 neuraminidase serotypes (N1-N11) have been identified, which are used to classify influenza viruses (e.g. H1N1). Influenza virus infections are most prevalent in winter. In annual influenza epidemics 5-15% of the population are affected with upper respiratory tract infections. Hospitalization and deaths mainly occur in high- risk groups. Annual epidemics are thought to result in between three and five million cases of severe illness and between 250,000 and 650,000 deaths every year around the world. The estimated costs of influenza epidemics to the US economy are estimated to be over 10 billion dollars per year, resulting from health care costs and lost productivity. Influenza type A viruses, which are typically isolated from wild birds, commonly infect human individuals. Influenza type A H3N2 viruses are particularly associated with high morbidity and mortality, and have a disproportionally adverse effect upon the public health. It was introduced into the human population in 1968 as an avian reassortant virus and caused the 1968 Hong Kong Flu (H3N2) pandemic with more than one million deaths worldwide. Since then, H3N2 viruses have rapidly evolved and have been responsible for many seasonal epidemics. The World Health Organization (WHO) recommends yearly vaccinations for high-risk groups. However, influenza viruses rapidly mutate as a result of immune selection pressure. Mutations in the influenza genome induce amino acid substitution(s) that cause antigenic changes in the HA and NA protein, referred to as antigenic drift. This frequently results in the escape of the virus from host immunity. Hence, influenza viruses have a remarkable ability to evolve and evade neutralization by antibodies elicited by prior infections or vaccinations. This antigenic evolution, or drift, is mainly caused by amino acid substitutions in the globular head of the HA protein where binding occurs with sialic acid receptors of host cells. These substitutions in circulating influenza viruses lead to antigenic differences as compared to employed vaccines, resulting in poor vaccine-mediated protection. Therefore, the WHO Global Influenza Surveillance and Response System (GISRS) continuously monitors antigenic changes in circulating influenza viruses and recommends updated compositions of influenza vaccines twice yearly. Antigenic surveillance and vaccine strain selection rely predominantly on the hemagglutination inhibition (HI) assay, in which the ability of serum antibodies to block erythrocyte receptor binding by an influenza virus HA protein is quantified. By blocking erythrocyte receptor binding, such serum antibodies prevent virus-mediated agglutination of erythrocytes and are measured as a correlate of protection. The HI assay is generally considered the assay of choice for global influenza surveillance and for determining the antigenic characteristics of influenza viral isolates. It is a traditional and very reliable test for typing and further characterizing of a given influenza viral isolate. This assay, which was originally developed in 1942 by the American virologist Georg Hirst, is based upon the tendency of the influenza HA protein to bind to erythrocytes, causing them to agglutinate. An HI assay is typically performed by incubating a test influenza virus, or HA protein or HA antigens thereof, with serum antibodies that are reactive against the HA proteins of a known reference influenza strain. Subsequently, erythrocytes are added. If the serum antibodies bind HA proteins or HA antigens of the test influenza virus, these HA proteins or antigens are no longer able to bind erythrocytes. As a result of the lack of binding, the erythrocytes do not agglutinate. Hence, inhibition of agglutination indicates that the test virus is similar to the reference strain, or that the test virus is an antigenically related strain that is able to be bound by the serum antibodies. The HI assay is useful for selecting virus strains that are antigenically representative of circulating viruses for vaccine development. Besides HI assays, hemagglutination assays (HA assays) are also widely used in the detection and characterization of influenza virus. An HA assay is typically performed by incubating a test influenza virus, or HA protein or HA antigens thereof, with erythrocytes. Binding of influenza HA proteins or HA antigens to the erythrocytes results in agglutination thereof, which is typically visible as a diffuse reddish solution. Contrary, when no agglutination takes place, the erythrocytes settle to the bottom of the well. The presence or absence of agglutination is thus indicative for whether or not an influenza strain or HA protein or HA antigen capable of binding the erythrocytes is present. Preferably, fowl erythrocytes like chicken or turkey erythrocytes are used in HI and HA assays, in view of their larger size as compared to mammalian erythrocytes, which is favorable for their application due to quicker precipitation. Smaller sized mammalian erythrocytes like human or guinea pig erythrocytes precipitate slower and delay the readout of the results. A/H3N2 viruses, which are associated with high morbidity and mortality, exhibit a particularly rapid antigenic drift and as a result the WHO has recommended 28 vaccine strain updates since these viruses started circulating in humans in 1968. In recent years, the rapid antigenic evolution of A/H3N2 viruses coincided with altered receptor usage, which in turn has resulted in their inability to agglutinate fowl erythrocytes. As a result, antigenic characterization of circulating A/H3N2 viruses using HI or HA assays is becoming increasingly difficult, complicating the selection of appropriate vaccine strains. The receptor- binding phenotype of recent A/H3N2 viruses is also hampering virus replication under laboratory conditions for amplification of clinical isolates and leads to adaptive substitutions when grown in embryonated chicken eggs. The difficulty to characterize circulating A/H3N2 viruses antigenically and the difficulty of large- scale virus production without egg-adaptation has led to serious problems with the production and effectiveness of A/H3N2 influenza vaccines. It is an object of the present invention to provide means and methods for detection, production, isolation and/or characterization of influenza viruses. Preferably, means and methods are provided that are suitable for detecting, producing, isolating and/or characterizing recently evolved A/H3N2 viruses that are difficult or impossible to characterize in currently used HI assays, such as for instance influenza viral strains A/NL/761/09, A/NL/2413/16, A/NL/751/17, A/NL/1797/17, A/NL/295/19, A/NL/314/19, A/NL/371/19, A/NL/03466/17, NIB-112 (A/Switzerland/8060/17), A/NL/10616/18, A/NL/622/12 and/or A/Switzerland/9715293/13. Alternatively, recently evolved A/H3N2 viruses can be identified by their clades. Such clades include clades 3C.1, 3C.2a1, 3C.2a1b, 3C.2a, 3C.2a2, 3C.3 and 3C.3a. In some embodiments the present invention provides a synthetic or recombinant cell or compound, comprising at its surface a multi-antennary glycan wherein at least one arm of said multi-antennary glycan comprises at least three N-acetyl lactosamine (LacNAc) moieties. Some preferred embodiments provide a synthetic or recombinant cell or compound comprising at its surface a multi- antennary glycan wherein a first arm of said multi-antennary glycan comprises at least one LacNAc moiety and wherein a second arm of said multi-antennary glycan comprises at least three LacNAc moieties. The present inventors have identified a minimal receptor that can be bound by many recent A/H3N2 viruses that are difficult or impossible to characterize in currently used HI and HA assays. Instead of using naturally occurring erythrocytes of various different origins in an attempt to perform an HI or HA assay for such recent A/H3N2 viruses, according to common approaches, the present inventors chose a different strategy. They prepared synthetic erythrocytes with modified multi-antennary N-glycans having different numbers of LacNAc repeating units, including asymmetrical architectures in which the various antennae were modified by oligo-LacNAc moieties of different lengths. While many recent A/H3N2 viruses are not able to bind and agglutinate natural fowl erythrocytes in current HI and HA assays, the present inventors have provided the insight that many of these recent A/H3N2 viruses bind cells, or non- cellular compounds, having at their surface a minimal receptor that comprises a multi-antennary glycan wherein at least one arm of said multi-antennary glycan comprises at least three LacNAc moieties. Glycan analysis of cell surface N-glycans of various natural fowl erythrocytes revealed an absence or very low abundance of this minimal receptor on natural fowl erythrocytes, explaining why currently used HI and HA assays fail to characterize many recent A/H3N2 viruses. Now that the insight of the present invention has been provided, it has become possible to use cells or non-cellular compounds comprising such minimal receptor for detection, production, isolation and/or characterization of such recent A/H3N2 viruses. For instance, in some embodiments an erythrocyte according to the invention comprising said minimal receptor is used in a HA or HI assay for detection and/or antigenic characterization of circulating influenza viruses. Whereas currently used HA and HI assays fail to detect many recently circulating A/H3N2 viruses such as for instance A/NL/761/09, A/NL/2413/16, A/NL/751/17, A/NL/1797/17, A/NL/295/19, A/NL/314/19, A/NL/371/19, A/NL/03466/17, NIB-112 (A/Switzerland/8060/17), A/NL/10616/18, A/NL/622/12 and A/Switzerland/9715293/13, these viruses can now be detected using erythrocytes according to the present invention, as shown in the Examples and in Table 2. Some embodiments therefore provide a cell or a compound, having at its surface a multi-antennary glycan wherein at least one arm of said multi-antennary glycan comprises at least three LacNAc moieties, and wherein said cell or compound is able to agglutinate in the presence of an A/H3N2 virus selected from the group consisting of A/NL/761/09, A/NL/2413/16, A/NL/751/17, A/NL/1797/17, A/NL/295/19, A/NL/314/19, A/NL/371/19, A/NL/03466/17, NIB-112 (A/Switzerland/8060/17), A/NL/10616/18, A/NL/622/12 and A/Switzerland/9715293/13, preferably selected from the group consisting of A/NL/1797/17 and A/NL/371/19. In some embodiments, said cell or compound comprises at its surface a multi-antennary glycan, wherein a first arm of said multi-antennary glycan comprises at least one LacNAc moiety and wherein a second arm of said multi-antennary glycan comprises at least three LacNAc moieties. Further provided is therefore a cell or compound, having at its surface a multi-antennary glycan wherein a first arm of said multi-antennary glycan comprises at least one LacNAc moiety and wherein a second arm of said multi- antennary glycan comprises at least three LacNAc moieties, and wherein said cell or compound is able to agglutinate in the presence of an A/H3N2 virus selected from the group consisting of A/NL/761/09, A/NL/2413/16, A/NL/751/17, A/NL/1797/17, A/NL/295/19, A/NL/314/19, A/NL/371/19, A/NL/03466/17, NIB-112 (A/Switzerland/8060/17), A/NL/10616/18, A/NL/622/12 and A/Switzerland/9715293/13, preferably selected from the group consisting of A/NL/1797/17 and A/NL/371/19. Some embodiments provide a cell or compound, having at its surface a multi-antennary glycan wherein at least one arm of said multi-antennary glycan comprises at least three LacNAc moieties, and wherein said multi-antennary glycan is present at the surface of said cell or compound at a surface density (also referred to as abundance) that is higher than the surface density (or abundance) of said multi-antennary glycan on natural turkey erythrocytes. Such high density typically allows for agglutination in the presence of an H3N2 virus selected from the group consisting of A/NL/761/09, A/NL/2413/16, A/NL/751/17, A/NL/1797/17, A/NL/295/19, A/NL/314/19, A/NL/371/19, A/NL/03466/17, NIB-112 (A/Switzerland/8060/17), A/NL/10616/18, A/NL/622/12 and A/Switzerland/9715293/13, contrary to the very low concentration of such multi-antennary glycan that is observed on natural erythrocytes, including natural fowl, guinea pig and human erythrocytes. Such low concentration on natural erythrocytes does not result in sufficient agglutination to perform HI or HA assays in the presence of A/NL/761/09, A/NL/2413/16, A/NL/751/17, A/NL/1797/17, A/NL/295/19, A/NL/314/19, A/NL/371/19, A/NL/03466/17, NIB-112 (A/Switzerland/8060/17), A/NL/10616/18, A/NL/622/12 or A/Switzerland/9715293/13. In some embodiments, said cell or compound comprises at its surface a multi-antennary glycan, wherein a first arm of said multi- antennary glycan comprises at least one LacNAc moiety and wherein a second arm of said multi-antennary glycan comprises at least three LacNAc moieties. Further provided is therefore a cell or compound, having at its surface a multi-antennary glycan wherein a first arm of said multi-antennary glycan comprises at least one LacNAc moiety and wherein a second arm of said multi-antennary glycan comprises at least three LacNAc moieties, and wherein said multi-antennary glycan is present at the surface of said cell or compound at a surface density (also referred to as abundance) that is higher than the surface density (or abundance) of said multi-antennary glycan on natural turkey erythrocytes. A cell according to the invention is preferably a synthetic or recombinant cell. As used herein, the term “synthetic or recombinant cell” means a cell that has been artificially modified. Such cell is therefore a non-native cell. As used herein, a non-native cell is also referred to as a non-natural cell. A synthetic or recombinant cell according to the present invention is a glycan-engineered cell, meaning that it has a different glycan pattern at its surface as compared to the natural, native glycan pattern of said cell. In some embodiments, the natural glycan pattern of said cell has been artificially altered by enzymatic engineering. Preferably, the natural glycan pattern of said cell has been artificially altered by exo-enzymatic cell surface glycan engineering. In preferred embodiments, the natural glycan pattern of said cell has been artificially altered by an enzymatic cell surface glycan engineering method according to the present invention. A cell according to the invention typically comprises a minimal receptor according to the present invention. A synthetic or recombinant cell according to the invention can also be referred to as a non-native cell according to the invention, or a non-natural cell according to the invention, or a glyco-engineered cell according to the invention, or a glycan-engineered cell according to the invention, or a glycan-remodelled cell according to the invention, or a glycan modified cell according to the invention. In some preferred embodiments, a cell according to the invention is an erythrocyte. As used herein, the term “synthetic or recombinant erythrocyte” means an erythrocyte that has been artificially modified. Such erythrocyte is therefore a non- native erythrocyte. A synthetic or recombinant erythrocyte according to the present invention is a glycan-engineered erythrocyte, meaning that it has a different glycan pattern at its surface as compared to the natural, native glycan pattern of said erythrocyte. In some embodiments, the natural glycan pattern of said erythrocyte has been artificially altered by enzymatic engineering, preferably by exo-enzymatic cell surface glycan engineering. In preferred embodiments, the natural glycan pattern of said erythrocyte has been artificially altered by an enzymatic cell surface glycan engineering method according to the present invention. In some embodiments, the natural glycan pattern of said erythrocyte has been artificially altered by extracting said erythrocyte from blood and subsequently subjecting said erythrocyte to an enzymatic cell surface glycan engineering method according to the present invention. An erythrocyte according to the invention typically comprises a minimal receptor according to the present invention. A synthetic or recombinant erythrocyte according to the invention can also be referred to as a non-native erythrocyte according to the invention, or a non-natural erythrocyte according to the invention, or a glyco-engineered erythrocyte according to the invention, or a glycan-engineered erythrocyte according to the invention, or a glycan-remodelled erythrocyte according to the invention, or a glycan modified erythrocyte according to the invention. In some embodiments, a synthetic or recombinant erythrocyte according to the present invention is a fowl erythrocyte, preferably a chicken or turkey erythrocyte. As explained above, fowl erythrocytes are preferred for HA and HI assays in view of their larger size as compared to mammalian erythrocytes, resulting in enhanced agglutination upon binding by influenza HA protein as compared to mammalian erythrocytes. Further provided is therefore a non-native fowl erythrocyte comprising at its surface a multi-antennary glycan wherein at least one arm of said multi-antennary glycan comprises at least three N-acetyl lactosamine (LacNAc) moieties. Some preferred embodiments provide a non-native fowl erythrocyte comprising at its surface a multi-antennary glycan wherein a first arm of said multi-antennary glycan comprises at least one LacNAc moiety and wherein a second arm of said multi-antennary glycan comprises at least three LacNAc moieties. In some embodiments, a cell according to the invention is a synthetic or recombinant erythrocyte precursor cell. A preferred example of an erythrocyte precursor cell is a reticulocyte. In some preferred embodiments, a cell according to the invention is an influenza production cell. As used herein, the term “influenza production cell” means a cell that is suitable to propagate influenza virus. In some embodiments, an influenza production cell according to the invention is a Madin-Darby canine kidney (MDCK) cell, an African green monkey kidney (Vero) cell, a PER.C6 cell, a Muscovy duck retina (AGE1.CR) cell, a PBG.PK2.1 cell, an EB66 cell, a DuckCelT- T17 cell, or a QOR/2E11 cell. These cells are suitable production cells for propagating influenza viruses. In a preferred embodiment, a cell according to the invention is an MDCK cell or a Vero cell. More preferably, a cell according to the invention is a Vero cell. In some embodiments, a cell according to the invention is a cell that is suitable for growing or replicating recently circulating A/H3N2 viruses, such as for instance A/NL/761/09, A/NL/2413/16, A/NL/751/17, A/NL/1797/17, A/NL/295/19, A/NL/314/19, A/NL/371/19, A/NL/03466/17, NIB-112 (A/Switzerland/8060/17), A/NL/10616/18, A/NL/622/12 and/or A/Switzerland/9715293/13. This is particularly advantageous for vaccine production. Some embodiments therefore provide a use of a cell according to the invention for production of an influenza virus. A use of a cell according to the invention for production of an influenza virus selected from the group consisting of A/NL/761/09, A/NL/2413/16, A/NL/751/17, A/NL/1797/17, A/NL/295/19, A/NL/314/19, A/NL/371/19, A/NL/03466/17, NIB-112 (A/Switzerland/8060/17), A/NL/10616/18, A/NL/622/12 and A/Switzerland/9715293/13 is also provided herewith. A cell according to the invention is preferably a cell that is suitable for growing or replicating recently circulating A/H3N2 viruses belonging to clade 3C.1, 3C.2a1, 3C.2a1b, 3C.2a, 3C.2a2, 3C.3 or 3C.3a. This is particularly advantageous for vaccine production, as influenza viruses of these clades are typically difficult to propagate in current production cells. Some embodiments therefore provide a use of a cell according to the invention for production of an influenza virus that belongs to clade 3C.1, 3C.2a1, 3C.2a1b, 3C.2a, 3C.2a2, 3C.3 or 3C.3a. Some embodiments provide a use according to the invention for production of an influenza virus, wherein an MDCK cell, a Vero cell, a PER.C6 cell, an AGE1.CR cell, a PBG.PK2.1 cell, an EB66 cell, a DuckCelT-T17 cell, or a QOR/2E11 cell is used. In a preferred embodiment, an MDCK cell or a Vero cell is used. More preferably, a Vero cell is used. In some embodiments, a compound according to the invention is a particle, or a bead, such as for instance a polystyrene bead, or a scaffold or a solid surface. In preferred embodiments, a compound according to the invention is a particle or a bead. Such particle or bead is for instance suitable for detecting influenza virus using flow cytometry, an agglutination assay, or an agglutination inhibition assay. A scaffold or a solid surface according to the invention is for instance suitable for site-specific binding, detection and/or characterization of an influenza virus, or for the detection of the ligand requirements of influenza viruses. As used herein, the term “glycan” has its usual meaning in the art. A glycan is typically a polysaccharide comprising two or more mono- or disaccharides that are glycosidically linked to each other. The term “N-glycan” (also referred to as N-linked glycan) also has its usual meaning in the art. Typically, a naturally occurring N-glycan is linked to an asparagine residue on the reducing end. N-acetyl lactosamine (LacNAc), also known as Galβ(1,4)GlcNAc and 2-(acetylamino)-2-deoxy-4-O-hexopyranosylhexopyranose, is a disaccharide consisting of galactose and N-acetylglucosamine. An “influenza A virus subtype” as used herein refers to an influenza A virus like for example H1N1, H1N2, H1N7, H2N2, H3N2, H3N8, H4N8, H5N1, H5N2, H5N9, H6N2, H6N5, H7N2, H7N3, H7N7, H8N4, H9N2, H10N7, H11N6, H12N5 or H13N6. Influenza subtypes can be further divided into different subgroups, referred to as clades. For instance, influenza A subtype H3N2 is divided into, amongst others, clades 3C.1, 3C.2a1, 3C.2a1b, 3C.2a, 3C.2a2, 3C.3 and 3C.3a. The term “influenza A virus strains” as used herein refers to different strains of a certain influenza A virus subtype. For instance, influenza strains A/NL/1797/17 and A/NL/371/19 are different strains that belong to the H3N2 subtype. Strain A/NL/1797/17 belongs to clade 3C.2a1, while strain A/NL/371/19 belongs to clade 3C.2a1b. As used herein, a multi-antennary glycan, wherein at least one arm of said multi-antennary glycan comprises at least three LacNAc moieties, is referred to as a minimal receptor according to the present invention. In some embodiments, said at least one arm of said multi-antennary glycan comprises at least three consecutive LacNAc moieties. In some preferred embodiments, said minimal receptor according to the present invention comprises a multi-antennary glycan, wherein a first arm of said multi-antennary glycan comprises at least one LacNAc moiety and wherein a second arm of said multi-antennary glycan comprises at least three LacNAc moieties, preferably at least three consecutive LacNAc moieties. In some embodiments, said multi-antennary glycan is a bi-antennary glycan, wherein a first arm of said bi-antennary glycan comprises at least one LacNAc moiety and wherein the second arm of said bi-antennary glycan comprises at least three LacNAc moieties, preferably at least three consecutive LacNAc moieties. In some embodiments, at least one arm of said minimal receptor comprises one or more additional moieties. In some embodiments a first arm of said multi- antennary glycan comprises at least one LacNAc moiety and further comprises one or more other mono- or disaccharide moieties, such as for instance one or more galactose, N-acetylglucosamine (GlcNAc), fucose and/or mannose moieties. In some embodiments said first arm comprises one or more additional LacNAc moieties, resulting in a first arm comprising two, three or even more LacNAc moieties. In some embodiments, said second arm of said multi-antennary glycan, comprising at least three LacNAc moieties, further comprises one or more other mono- or disaccharide moieties, such as for instance one or more galactose, GlcNAc, fucose and/or mannose moieties. For instance, one or more non-LacNAc moieties may be present between one LacNAc moiety and another LacNAc moiety of said second arm. Alternatively, one or more non-LacNAc moieties may be coupled to three consecutive LacNAc moieties. In some embodiments, said second arm comprises one or more additional LacNAc moieties, resulting in a second arm comprising four, five or even more LacNAc moieties. In some embodiments, said second arm of said multi-antennary glycan comprises at least four consecutive LacNAc moieties. In some embodiments, said second arm of said multi-antennary glycan comprises at least five consecutive LacNAc moieties. Some embodiments provide a cell or compound according to the invention, having at its surface a multi-antennary glycan, wherein a first arm of said multi- antennary glycan comprises one LacNAc moiety and wherein a second arm of said multi-antennary glycan comprises three LacNAc moieties, preferably three consecutive LacNAc moieties. Preferably, said second arm of said multi-antennary glycan, comprising three LacNAc moieties, is 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, both arms of said bi-antennary glycan are 2,6-sialylated. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound according to the invention, having at its surface a multi-antennary glycan, wherein a first arm of said multi- antennary glycan comprises two LacNAc moieties, preferably two consecutive LacNAc moieties, and wherein a second arm of said multi-antennary glycan comprises three LacNAc moieties, preferably three consecutive LacNAc moieties. Preferably, said second arm of said multi-antennary glycan, comprising three LacNAc moieties, is 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, both arms of said bi-antennary glycan are 2,6-sialylated. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound according to the invention, having at its surface a multi-antennary glycan, wherein a first arm of said multi- antennary glycan comprises three LacNAc moieties, preferably three consecutive LacNAc moieties, and wherein a second arm of said multi-antennary glycan comprises three LacNAc moieties, preferably three consecutive LacNAc moieties. Preferably, said second arm of said multi-antennary glycan, comprising three LacNAc moieties, is 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, both arms of said bi-antennary glycan are 2,6-sialylated. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound according to the invention, having at its surface a multi-antennary glycan, wherein a first arm of said multi- antennary glycan comprises one LacNAc moiety and wherein a second arm of said multi-antennary glycan comprises four LacNAc moieties, preferably four consecutive LacNAc moieties. Preferably, said second arm of said multi-antennary glycan, comprising four LacNAc moieties, is 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, both arms of said bi-antennary glycan are 2,6-sialylated. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound according to the invention, having at its surface a multi-antennary glycan, wherein a first arm of said multi- antennary glycan comprises two LacNAc moieties, preferably two consecutive LacNAc moieties, and wherein a second arm of said multi-antennary glycan comprises four LacNAc moieties, preferably four consecutive LacNAc moieties. Preferably, said second arm of said multi-antennary glycan, comprising four LacNAc moieties, is 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, both arms of said bi-antennary glycan are 2,6-sialylated. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound according to the invention, having at its surface a multi-antennary glycan, wherein a first arm of said multi- antennary glycan comprises three LacNAc moieties, preferably three consecutive LacNAc moieties, and wherein a second arm of said multi-antennary glycan comprises four LacNAc moieties, preferably four consecutive LacNAc moieties. Preferably, said second arm of said multi-antennary glycan, comprising four LacNAc moieties, is 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, both arms of said bi-antennary glycan are 2,6-sialylated. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound according to the invention, having at its surface a multi-antennary glycan, wherein a first arm of said multi- antennary glycan comprises four LacNAc moieties, preferably four consecutive LacNAc moieties, and wherein a second arm of said multi-antennary glycan comprises four LacNAc moieties, preferably four consecutive LacNAc moieties. Preferably, said second arm of said multi-antennary glycan, comprising four LacNAc moieties, is 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, both arms of said bi-antennary glycan are 2,6-sialylated. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound according to the invention, having at its surface a multi-antennary glycan, wherein a first arm of said multi- antennary glycan comprises one LacNAc moiety and wherein a second arm of said multi-antennary glycan comprises five LacNAc moieties, preferably five consecutive LacNAc moieties. Preferably, said second arm of said multi-antennary glycan, comprising five LacNAc moieties, is 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, both arms of said bi-antennary glycan are 2,6-sialylated. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound according to the invention, having at its surface a multi-antennary glycan, wherein a first arm of said multi- antennary glycan comprises two LacNAc moieties, preferably two consecutive LacNAc moieties, and wherein a second arm of said multi-antennary glycan comprises five LacNAc moieties, preferably five consecutive LacNAc moieties. Preferably, said second arm of said multi-antennary glycan, comprising five LacNAc moieties, is 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, both arms of said bi-antennary glycan are 2,6-sialylated. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound according to the invention, having at its surface a multi-antennary glycan, wherein a first arm of said multi- antennary glycan comprises three LacNAc moieties, preferably three consecutive LacNAc moieties, and wherein a second arm of said multi-antennary glycan comprises five LacNAc moieties, preferably five consecutive LacNAc moieties. Preferably, said second arm of said multi-antennary glycan, comprising five LacNAc moieties, is 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, both arms of said bi-antennary glycan are 2,6-sialylated. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound according to the invention, having at its surface a multi-antennary glycan, wherein a first arm of said multi- antennary glycan comprises four LacNAc moieties, preferably four consecutive LacNAc moieties, and wherein a second arm of said multi-antennary glycan comprises five LacNAc moieties, preferably five consecutive LacNAc moieties. Preferably, said second arm of said multi-antennary glycan, comprising five LacNAc moieties, is 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, both arms of said bi-antennary glycan are 2,6-sialylated. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound according to the invention, having at its surface a multi-antennary glycan, wherein a first arm of said multi- antennary glycan comprises five LacNAc moieties, preferably five consecutive LacNAc moieties, and wherein a second arm of said multi-antennary glycan comprises five LacNAc moieties, preferably five consecutive LacNAc moieties. Preferably, said second arm of said multi-antennary glycan, comprising five LacNAc moieties, is 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, both arms of said bi-antennary glycan are 2,6-sialylated. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound according to the invention having at its surface a multi-antennary glycan, wherein a first arm of said multi- antennary glycan comprises at least one LacNAc moiety and wherein a second arm of said multi-antennary glycan comprises at least three LacNAc moieties, and wherein said first and second arms are bound via one or more mannose moieties and/or one or more GlcNAc moieties to the surface of said cell or compound according to the invention. In some embodiments, said first and second arms are bound to mannose moieties, which mannose moieties are bound to a third, branch- forming, mannose moiety via an α-1,3 linkage and an α-1,6 linkage. This branch- forming mannose moiety is in some embodiments bound via one or two GlcNAc moieties to said cell or compound, similar to the core pentasaccharides found in N-linked glycans of natural erythrocytes. Some embodiments provide a cell or compound according to the invention having at its surface a multi-antennary glycan, wherein a first arm of said multi- antennary glycan comprises at least one LacNAc moiety and wherein a second arm of said multi-antennary glycan comprises at least three LacNAc moieties, and wherein said first and second arms are bound to a branched pentasaccharide. Said pentasaccharide preferably comprises a branching point resulting in at least two branches, wherein one branch comprises said first arm and the other branch comprises said second arm. In some embodiments said pentasaccharide comprises one or more GlcNAc moieties and one or more mannoses. In some embodiments, said branched pentasaccharide consists of two linked GlcNAc moieties, which are linked to a branch-forming mannose, which in turn is linked via an α-1,3 linkage and an α-1,6 linkage to two other mannose moieties, thereby forming two branches which comprise the first and second arms of a minimal receptor according to the invention. In some embodiments, said branched pentasaccharide is bound to said cell or compound via the reducing end GlcNAc of said two linked GlcNAc moieties. In some embodiments, said branched pentasaccharide consists of a GlcNAc moiety that is linked via a β-1,4 linkage to another GlcNAc moiety, which is linked via a β- 1,4 linkage to a mannose moiety, which is linked via an α-1,3 linkage and an α-1,6 linkage to two other mannose moieties, thereby forming two branches. In some embodiments, said pentasaccharide is similar to the core pentasaccharides found in N-linked glycans that are present on natural erythrocytes. Such pentasaccharides consist of two N-acetylglucosamines and three mannoses, forming a branching point, as shown in Figure 4. In some embodiments, at least one arm of a minimal receptor according to the present invention is 2,6-sialylated. In some preferred embodiments, the arm of said multi-antennary glycan that comprises at least three LacNAc moieties is 2,6- sialylated. In some embodiments, all arms of said multi-antennary glycan are 2,6- sialylated. Minimal receptors according to the present invention wherein at least the arm that comprises at least three LacNAc moieties is 2,6-sialylated are particularly well bound by many recently circulating A/H3N2 strains. Some embodiments provide a cell or compound having at its surface a multi- antennary glycan, wherein said glycan comprises a branched pentasaccharide consisting of a GlcNAc moiety that is linked via a β-1,4 linkage to another GlcNAc moiety, which is linked via a β-1,4 linkage to a mannose moiety, which is linked via an α-1,3 linkage and an α-1,6 linkage to two other mannose moieties, thereby forming two branches, wherein three LacNAc moieties are linked to at least one of the branches, forming an arm. In some preferred embodiments, said arm is 2,6- sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, both arms of said bi-antennary glycan are 2,6- sialylated. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound having at its surface a multi- antennary glycan, wherein said glycan comprises a branched pentasaccharide consisting of a GlcNAc moiety that is linked via a β-1,4 linkage to another GlcNAc moiety, which is linked via a β-1,4 linkage to a mannose moiety, which is linked via an α-1,3 linkage and an α-1,6 linkage to two other mannose moieties, thereby forming two branches, wherein one LacNAc moiety is linked to the first branch, forming a first arm, and three LacNAc moieties are linked to the second branch, forming a second arm. In some preferred embodiments, said second arm is 2,6- sialylated. In some embodiments, both arms are 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some preferred embodiments, said multi-antennary glycan is represented by Compound 15 or 17 of Figure 4. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound having at its surface a multi- antennary glycan, wherein said glycan comprises a branched pentasaccharide consisting of a GlcNAc moiety that is linked via a β-1,4 linkage to another GlcNAc moiety, which is linked via a β-1,4 linkage to a mannose moiety, which is linked via an α-1,3 linkage and an α-1,6 linkage to two other mannose moieties, thereby forming two branches, wherein two LacNAc moieties are linked to the first branch, forming a first arm, and three LacNAc moieties are linked to the second branch, forming a second arm. In some preferred embodiments, said second arm is 2,6- sialylated. In some embodiments, both arms are 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound having at its surface a multi- antennary glycan, wherein said glycan comprises a branched pentasaccharide consisting of a GlcNAc moiety that is linked via a β-1,4 linkage to another GlcNAc moiety, which is linked via a β-1,4 linkage to a mannose moiety, which is linked via an α-1,3 linkage and an α-1,6 linkage to two other mannose moieties, thereby forming two branches, wherein three LacNAc moieties are linked to the first branch, forming a first arm, and three LacNAac moieties are linked to the second branch, forming a second arm. In some embodiments, at least one of said arms is 2,6-sialylated. In some embodiments, both arms are 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some preferred embodiments, said multi-antennary glycan is represented by Compound 14 of Figure 4. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound having at its surface a multi- antennary glycan, wherein said glycan comprises a branched pentasaccharide consisting of a GlcNAc moiety that is linked via a β-1,4 linkage to another GlcNAc moiety, which is linked via a β-1,4 linkage to a mannose moiety, which is linked via an α-1,3 linkage and an α-1,6 linkage to two other mannose moieties, thereby forming two branches, wherein one LacNAc moiety is linked to the first branch, forming a first arm, and four LacNAc moieties are linked to the second branch, forming a second arm. In some preferred embodiments, said second arm is 2,6- sialylated. In some embodiments, both arms are 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound having at its surface a multi- antennary glycan, wherein said glycan comprises a branched pentasaccharide consisting of a GlcNAc moiety that is linked via a β-1,4 linkage to another GlcNAc moiety, which is linked via a β-1,4 linkage to a mannose moiety, which is linked via an α-1,3 linkage and an α-1,6 linkage to two other mannose moieties, thereby forming two branches, wherein two LacNAc moieties are linked to the first branch, forming a first arm, and four LacNAc moieties are linked to the second branch, forming a second arm. In some preferred embodiments, said second arm is 2,6- sialylated. In some embodiments, both arms are 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound having at its surface a multi- antennary glycan, wherein said glycan comprises a branched pentasaccharide consisting of a GlcNAc moiety that is linked via a β-1,4 linkage to another GlcNAc moiety, which is linked via a β-1,4 linkage to a mannose moiety, which is linked via an α-1,3 linkage and an α-1,6 linkage to two other mannose moieties, thereby forming two branches, wherein three LacNAc moieties are linked to the first branch, forming a first arm, and four LacNAc moieties are linked to the second branch, forming a second arm. In some embodiments, at least one of said arms is 2,6-sialylated. In some embodiments, said first arm is 2,6-sialylated. In some embodiments, said second arm is 2,6-sialylated. In some embodiments, both arms are 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound having at its surface a multi- antennary glycan, wherein said glycan comprises a branched pentasaccharide consisting of a GlcNAc moiety that is linked via a β-1,4 linkage to another GlcNAc moiety, which is linked via a β-1,4 linkage to a mannose moiety, which is linked via an α-1,3 linkage and an α-1,6 linkage to two other mannose moieties, thereby forming two branches, wherein four LacNAc moieties are linked to the first branch, forming a first arm, and four LacNAc moieties are linked to the second branch, forming a second arm. In some embodiments, at least one of said arms is 2,6- sialylated. In some embodiments, both arms are 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound having at its surface a multi- antennary glycan, wherein said glycan comprises a branched pentasaccharide consisting of a GlcNAc moiety that is linked via a β-1,4 linkage to another GlcNAc moiety, which is linked via a β-1,4 linkage to a mannose moiety, which is linked via an α-1,3 linkage and an α-1,6 linkage to two other mannose moieties, thereby forming two branches, wherein one LacNAc moiety is linked to the first branch, forming a first arm, and five LacNAc moieties are linked to the second branch, forming a second arm. In some preferred embodiments, said second arm is 2,6- sialylated. In some embodiments, both arms are 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound having at its surface a multi- antennary glycan, wherein said glycan comprises a branched pentasaccharide consisting of a GlcNAc moiety that is linked via a β-1,4 linkage to another GlcNAc moiety, which is linked via a β-1,4 linkage to a mannose moiety, which is linked via an α-1,3 linkage and an α-1,6 linkage to two other mannose moieties, thereby forming two branches, wherein two LacNAc moieties are linked to the first branch, forming a first arm, and five LacNAc moieties are linked to the second branch, forming a second arm. In some preferred embodiments, said second arm is 2,6- sialylated. In some embodiments, both arms are 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound having at its surface a multi- antennary glycan, wherein said glycan comprises a branched pentasaccharide consisting of a GlcNAc moiety that is linked via a β-1,4 linkage to another GlcNAc moiety, which is linked via a β-1,4 linkage to a mannose moiety, which is linked via an α-1,3 linkage and an α-1,6 linkage to two other mannose moieties, thereby forming two branches, wherein three LacNAc moieties are linked to the first branch, forming a first arm, and five LacNAc moieties are linked to the second branch, forming a second arm. In some embodiments, at least one of said arms is 2,6-sialylated. In some embodiments, said first arm is 2,6-sialylated. In some embodiments, said second arm is 2,6-sialylated. In some embodiments, both arms are 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound having at its surface a multi- antennary glycan, wherein said glycan comprises a branched pentasaccharide consisting of a GlcNAc moiety that is linked via a β-1,4 linkage to another GlcNAc moiety, which is linked via a β-1,4 linkage to a mannose moiety, which is linked via an α-1,3 linkage and an α-1,6 linkage to two other mannose moieties, thereby forming two branches, wherein four LacNAc moieties are linked to the first branch, forming a first arm, and five LacNAc moieties are linked to the second branch, forming a second arm. In some embodiments, at least one of said arms is 2,6- sialylated. In some embodiments, said first arm is 2,6-sialylated. In some embodiments, said second arm is 2,6-sialylated. In some embodiments, both arms are 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. Some embodiments provide a cell or compound having at its surface a multi- antennary glycan, wherein said glycan comprises a branched pentasaccharide consisting of a GlcNAc moiety that is linked via a β-1,4 linkage to another GlcNAc moiety, which is linked via a β-1,4 linkage to a mannose moiety, which is linked via an α-1,3 linkage and an α-1,6 linkage to two other mannose moieties, thereby forming two branches, wherein five LacNAc moieties are linked to the first branch, forming a first arm, and five LacNAc moieties are linked to the second branch, forming a second arm. In some embodiments, at least one of said arms is 2,6- sialylated. In some embodiments, both arms are 2,6-sialylated. In some embodiments, said multi-antennary glycan is a bi-antennary glycan. In some embodiments, said cell is a synthetic or recombinant erythrocyte or influenza production cell, such as an MDCK cell or Vero cell. Preferably, said cell is a synthetic or recombinant erythrocyte. The invention further pertains to the production of glyco-engineered cells according to the present invention. Despite of the difficulties encountered in the art when enzymatically growing glycans on cell surfaces, the present inventors have succeeded in developing an exo-enzymatic cell surface glycan remodeling strategy to install minimal receptors according to the invention on cells. This strategy involves the use of a neuraminidase in order to desialylate cell surface glycans of natural cells, thereby revealing terminal galactosides which are appropriate receptors for additional LacNAc moieties. Further, the cells are incubated with a galactosyl transferase and an N-acetylglucosaminyl transferase that are capable of generating type II LacNAc structures, in the presence of uridine diphosphate galactose (UDP-Gal) and uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), in order to add additional LacNAc moieties to the cell surface glycans of said cells. The resulting synthetic cells, having various extended LacNAc moieties, are incubated with a 2,6-sialyltransferase and with cytidine-5'-monophosphate-N- acetylneuraminic acid (CMP-Neu5Ac) in order to install terminal α2,6-linked sialosides. With this method according to the invention, synthetic cells are generated that have a sufficiently high concentration at their surface of a minimal receptor according to the present invention to enable agglutination by recently circulating A/H3N2 viruses such as A/NL/761/09, A/NL/2413/16, A/NL/751/17, A/NL/1797/17, A/NL/295/19, A/NL/314/19, A/NL/371/19, A/NL/03466/17, NIB-112 (A/Switzerland/8060/17), A/NL/10616/18, A/NL/622/12 and A/Switzerland/9715293/13, contrary to natural erythrocytes. Accordingly, some embodiments provide a method for modifying cell surface glycans of glycan-containing cells, wherein the method comprises the steps of: - incubating glycan-containing cells with a neuraminidase; - incubating said cells with a galactosyl transferase and an N-acetyl glucosaminyl transferase that are capable of generating type II LacNAc structures, with uridine diphosphate galactose (UDP-Gal), and with uridine diphosphate N-acetylglucosamine (UDP-GlcNAc); and - incubating said cells with a 2,6-sialyltransferase and with cytidine-5'- monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac). In some embodiments, said glycan-containing cells are simultaneously incubated with said neuraminidase, with said galactosyl transferase and with said N-acetyl glucosaminyl transferase. In preferred embodiments said cells are incubated with said neuraminidase first, followed by incubation with said galactosyl transferase and said N-acetyl glucosaminyl transferase. Said cells are preferably incubated with said 2,6-sialyltransferase after incubation with said neuraminidase, galactosyl transferase and N-acetyl glucosaminyl transferase. The term “type II LacNAc structure” as used herein refers to an N-acetyl lactosamine structure composed of a galactose linked in a β1-4 linkage to an N-acetyl glucosamine. Some embodiments provide a method according to the invention for modifying cell surface glycans of glycan-containing cells, wherein said glycan- containing cells are erythrocytes or erythrocyte precursor cells. A preferred example of an erythrocyte precursor cell is a reticulocyte. In preferred embodiments, the cell surface glycans of erythrocytes are modified with a method according to the invention. Accordingly, some embodiments provide a method for modifying cell surface glycans of erythrocytes, comprising the steps of: - incubating erythrocytes with a neuraminidase; - incubating said erythrocytes with a galactosyl transferase and an N-acetyl glucosaminyl transferase that are capable of generating type II LacNAc structures, with uridine diphosphate galactose (UDP-Gal), and with uridine diphosphate N- acetylglucosamine (UDP-GlcNAc); and - incubating said erythrocytes with a 2,6-sialyltransferase and with cytidine-5'- monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac). In some embodiments, said erythrocytes are fowl erythrocytes, preferably chicken and/or turkey erythrocytes. In some embodiments, said erythrocytes are simultaneously incubated with said neuraminidase, with said galactosyl transferase and with said N-acetyl glucosaminyl transferase. In preferred embodiments said erythrocytes are incubated with said neuraminidase first, followed by incubation with said galactosyl transferase and said N-acetyl glucosaminyl transferase. Said erythrocytes are preferably incubated with said 2,6-sialyltransferase after incubation with said neuraminidase, galactosyl transferase and N-acetyl glucosaminyl transferase. Some embodiments provide an ex vivo method of extracting erythrocytes from a blood sample of a non-human animal and modifying the cell surface glycans of the extracted erythrocytes with a method according to the present invention. Further provided is therefore a method for modifying cell surface glycans of erythrocytes, comprising the steps of: a) obtaining erythrocytes from blood; b) incubating erythrocytes obtained in step a) with a neuraminidase; c) incubating said erythrocytes with a galactosyl transferase and an N-acetyl glucosaminyl transferase that are capable of generating type II LacNAc structures, with uridine diphosphate galactose (UDP-Gal), and with uridine diphosphate N- acetylglucosamine (UDP-GlcNAc); and d) incubating said erythrocytes with a 2,6-sialyltransferase and with cytidine-5'- monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac). In some embodiments said erythrocytes are simultaneously incubated with said neuraminidase, with said galactosyl transferase and with said N-acetyl glucosaminyl transferase. In preferred embodiments said erythrocytes are incubated with said neuraminidase first, followed by incubation with said galactosyl transferase and said N-acetyl glucosaminyl transferase. Said erythrocytes are preferably incubated with said 2,6-sialyltransferase after incubation with said neuraminidase, galactosyl transferase and N-acetyl glucosaminyl transferase. The invention further pertains to the production of glyco-engineered Vero cells with a method according to the present invention. Accordingly, some embodiments provide a method for modifying cell surface glycans of Vero cells, comprising the steps of: - incubating Vero cells with a neuraminidase; - incubating said Vero cells with a galactosyl transferase and an N-acetyl glucosaminyl transferase that are capable of generating type II LacNAc structures, with uridine diphosphate galactose (UDP-Gal), and with uridine diphosphate N- acetylglucosamine (UDP-GlcNAc); and - incubating said Vero cells with a 2,6-sialyltransferase and with cytidine-5'- monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac). The invention further pertains to the production of glyco-engineered MDCK cells with a method according to the present invention. Accordingly, some embodiments provide a method for modifying cell surface glycans of MDCK cells, comprising the steps of: - incubating MDCK cells with a neuraminidase; - incubating said MDCK cells with a galactosyl transferase and an N-acetyl glucosaminyl transferase that are capable of generating type II LacNAc structures, with uridine diphosphate galactose (UDP-Gal), and with uridine diphosphate N- acetylglucosamine (UDP-GlcNAc); and - incubating said MDCK cells with a 2,6-sialyltransferase and with cytidine-5'- monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac). In some embodiments said cells are simultaneously incubated with said neuraminidase, with said galactosyl transferase and with said N-acetyl glucosaminyl transferase. In preferred embodiments said cells are incubated with said neuraminidase first, followed by incubation with said galactosyl transferase and said N-acetyl glucosaminyl transferase. Said cells are preferably incubated with said 2,6-sialyltransferase after incubation with said neuraminidase, galactosyl transferase and N-acetyl glucosaminyl transferase. A method according to the invention is preferred over cell surface glycan- engineering methods as described in the art. For instance, Peng et al. (2017) describe the insertion of synthetic glycans into the cell membrane of MDCK cells using lipid anchors. To this end, glycans were first prepared synthetically. Subsequently, lipid anchors were coupled to these synthetic glycans, where after the resulting lipid-conjugated glycans were inserted into the membranes of MDCK cells. This method according to Peng et al. thus involves separate, artificial, synthesis and coupling steps, which is more complex and therefore more time consuming as compared to the methods according to the present invention. Furthermore, as the glycans of Peng et al. are artificially synthesized and subsequently coupled to lipid anchors, the glycosylation pattern of the resulting MDCK cells is typically quite different from natural glycosylation patterns. As the methods according to the invention involve the production of extended glycan structures by naturally occurring enzymes, a method according to the invention typically results in cells with a glycosylation pattern that more closely resembles the glycosylation pattern of natural cells. In Hong et al. (2019), a Pasteurella multocida α2-3-sialyltransferase M144D mutant, a Photobacterium damsela α2-6-sialyltransferase and a Helicobacter mustelae α1-2-fucosyltransferase were identified as efficient tools for live-cell glycan modification. MDCK cells were incubated with these enzymes and subsequently used to assay influenza A virus infection by several H3N2 and H1N1 strains. With these enzymes, additional fucose moieties were transferred to the N-glycans of the MDCK cells, thereby creating new sialyl Lewis X (sLe ^X ) epitopes, or additional sialic acid (N-acetylneuraminic acid; Neu5Ac) residues were added to Gal residues, thereby increasing cell-surface NeuAcα2-6-Gal epitopes. Of note, the above mentioned enzymes do not increase the number of LacNAc moieties of the glycan arms. This was not necessary because the H3N2 strains used in Hong et al. are older A/H3N2 viruses (A/Aichi/2/1968, A/Perth/16/2009). These older strains do not have the altered receptor usage of recent strains that led to the inability of the recent strains to agglutinate fowl erythrocytes, thereby hampering the current HI and HAI assays. The older H3N2 strains used by Hong et al. do therefore not require a minimal receptor according to the present invention, having a multi-antennary glycan wherein at least one arm comprises at least three LacNAc moieties and wherein another arm preferably comprises at least one LacNAc moiety. Hence, a minimal receptor according to the present invention is not produced, nor discussed, by Hong et al. Instead, Hong et al. discuss that influenza A virus (IAV)-dependent cell killing was enhanced by increasing the number of cell-surface NeuAcα2-6-Gal epitopes. In addition, Hong et al. also discuss that the severity of IAV infection is exacerbated by increasing the quantity of sialyl Lewix X (sLe ^X ) on the cell surface in a dose-dependent matter. Nothing in Hong et al. suggests that the number of the LacNAc moieties of the cell surface N-glycans influences the capacity of A/H3N2 viruses to bind. Some embodiments provide a method according to the present invention wherein said neuraminidase is an Arthrobacter Ureafaciens neuraminidase. In some embodiments, a method according to the present invention is provided wherein said galactosyl transferase is galactosyltransferase B4GalT1. In some embodiments, a method according to the present invention is provided wherein said N-acetyl glucosaminyl transferase is N-acetyl glucosaminyl transferase B3GNT2. In some embodiments, a method according to the present invention is provided wherein said 2,6-sialyl transferase is ST6Gal1. As shown in the Examples, these enzymes are particularly suitable for producing glyco-engineered cells according to the present invention, such as for instance glyco-engineered erythrocytes that are able to be agglutinated by recently evolved A/H3N2 viruses like A/NL/761/09, A/NL/2413/16, A/NL/751/17, A/NL/1797/17, A/NL/295/19, A/NL/314/19, A/NL/371/19, A/NL/03466/17, NIB-112 (A/Switzerland/8060/17), A/NL/10616/18, A/NL/622/12 and A/Switzerland/9715293/13. Further provided is therefore a method for modifying cell surface glycans of glycan-containing cells, wherein the method comprises the steps of: - incubating glycan-containing cells with an Arthrobacter Ureafaciens neuraminidase; - incubating said cells with galactosyltransferase B4GalT1, with N-acetyl glucosaminyl transferase B3GNT2, with uridine diphosphate galactose (UDP-Gal), and with uridine diphosphate N-acetylglucosamine (UDP-GlcNAc); and - incubating said cells with ST6Gal1 and with cytidine-5'-monophosphate-N- acetylneuraminic acid (CMP-Neu5Ac). In preferred embodiments, the cell surface glycans of erythrocytes or of erythrocyte precursor cells or of influenza production cells, such as MDCK cells or Vero cells, are modified with a method according to the invention. A preferred example of an erythrocyte precursor cell is a reticulocyte. Further provided is therefore a method for modifying cell surface glycans of erythrocytes or of erythrocyte precursor cells, the method comprising the steps of: - incubating erythrocytes or erythrocyte precursor cells with an Arthrobacter Ureafaciens neuraminidase; - incubating said erythrocytes or erythrocyte precursor cells with galactosyltransferase B4GalT1, with N-acetyl glucosaminyl transferase B3GNT2, with uridine diphosphate galactose (UDP-Gal), and with uridine diphosphate N- acetylglucosamine (UDP-GlcNAc); and - incubating said erythrocytes or erythrocyte precursor cells with ST6Gal1 and with cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac). Further provided is a method for modifying cell surface glycans of MDCK cells, the method comprising the steps of: - incubating MDCK cells with an Arthrobacter Ureafaciens neuraminidase; - incubating said MDCK cells with galactosyltransferase B4GalT1, with N-acetyl glucosaminyl transferase B3GNT2, with uridine diphosphate galactose (UDP-Gal), and with uridine diphosphate N-acetylglucosamine (UDP-GlcNAc); and - incubating said MDCK cells with ST6Gal1 and with cytidine-5'-monophosphate- N-acetylneuraminic acid (CMP-Neu5Ac). Further provided is therefore a method for modifying cell surface glycans of Vero cells, the method comprising the steps of: - incubating Vero cells with an Arthrobacter Ureafaciens neuraminidase; - incubating said Vero cells with galactosyltransferase B4GalT1, with N-acetyl glucosaminyl transferase B3GNT2, with uridine diphosphate galactose (UDP-Gal), and with uridine diphosphate N-acetylglucosamine (UDP-GlcNAc); and - incubating said Vero cells with ST6Gal1 and with cytidine-5'-monophosphate-N- acetylneuraminic acid (CMP-Neu5Ac). Before the present invention it was not expected that a method according to the present invention for enzymatically modifying N-glycans, comprising multiple steps of enzymatic modification with the above mentioned enzymes, can successfully be performed on cells, because enzymatic reactions are usually more efficient in a solution or suspension wherein their substrates are easily accessible. Glycans present on the cell membrane of cells are typically much less accessible for enzymes. However, the present inventors have shown in the Examples that an enzymatic method according to the present invention can be successfully performed on glycans present on the surface of cells. Besides the addition of enzymes to cells, it is also possible to genetically engineer cells such that the cells express, or overexpress, the required enzymes. In some embodiments, therefore, glycan-containing cells are genetically modified to express enzymes capable of producing minimal receptors according to the invention on the cell surface. In some embodiments, glycan-containing cells are genetically modified to express a galactosyl transferase and an N-acetylglucosaminyl transferase that are capable of generating type II LacNAc structures. In some embodiments, said glycan-containing cells are genetically modified to also express a 2,6-sialyltransferase. Hence, in some embodiments, glycan-containing cells are genetically modified to express: - a galactosyl transferase and an N-acetylglucosaminyl transferase that are capable of generating type II LacNAc structures, preferably in the presence of uridine diphosphate galactose (UDP-Gal) and uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), and - a 2,6-sialyltransferase that is capable of installing terminal α2,6-linked sialosides, preferably in the presence of cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac). Methods of genetically modifying cells in order to express nucleic acid molecules of interest are known in the art. Accordingly, some embodiments provide a method for modifying cell surface glycans of glycan-containing cells, wherein the method comprises the steps of: - providing glycan-containing cells with nucleic acid molecules encoding a galactosyl transferase and an N-acetylglucosaminyl transferase that are capable of generating type II LacNAc structures; and - culturing the resulting cells, preferably in the presence of uridine diphosphate galactose (UDP-Gal) and/or uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), resulting in cells with extended glycan structures. In some embodiments, a 2,6-sialyltransferase is added to the growing cells, either at the start of the culture or later, for instance 30 minutes, or 1 hour, or 2 hours, or 5 hours, or 10 hours, or 24 hours, or more than 24 hours, after the start of the culture. In some embodiments, a 2,6-sialyltransferase and cytidine-5'- monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac) are added to the growing cells. Alternatively, said cells are provided with a nucleic acid molecule encoding a 2,6-sialyltransferase. Accordingly, some embodiments provide a method for modifying cell surface glycans of glycan-containing cells, wherein the method comprises the steps of: - providing glycan-containing cells with nucleic acid molecules encoding a galactosyl transferase and an N-acetylglucosaminyl transferase that are capable of generating type II LacNAc structures; - providing said cells with a nucleic acid molecule encoding a 2,6- sialyltransferase; and - culturing the resulting cells, preferably in the presence of uridine diphosphate galactose (UDP-Gal), uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) and/or cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac), resulting in cells with extended glycan structures. In some embodiments, said glycan-containing cells are influenza production cells, such as MDCK cells or Vero cells. Also provided is therefore a method for modifying cell surface glycans of influenza production cells, preferably MDCK cells or Vero cells, wherein the method comprises the steps of: - providing influenza production cells, preferably MDCK cells or Vero cells, with nucleic acid molecules encoding a galactosyl transferase and an N-acetylglucosaminyl transferase that are capable of generating type II LacNAc structures; and - culturing the resulting cells, preferably in the presence of uridine diphosphate galactose (UDP-Gal) and/or uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), resulting in influenza production cells, preferably MDCK cells or Vero cells, with extended glycan structures. In some embodiments, a 2,6-sialyltransferase is added to the growing cells, either at the start of the culture or later, for instance 30 minutes, or 1 hour, or 2 hours, or 5 hours, or 10 hours, or 24 hours, or more than 24 hours, after the start of the culture. In some embodiments, a 2,6-sialyltransferase and cytidine-5'- monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac) are added to the growing cells. Alternatively, said cells are provided with a nucleic acid molecule encoding a 2,6-sialyltransferase. Accordingly, some embodiments provide a method for modifying cell surface glycans of influenza production cells, preferably MDCK cells or Vero cells, wherein the method comprises the steps of: - providing influenza production cells, preferably MDCK cells or Vero cells, with nucleic acid molecules encoding a galactosyl transferase and an N-acetylglucosaminyl transferase that are capable of generating type II LacNAc structures; - providing said cells with a nucleic acid molecule encoding a 2,6-sialyltransferase; and - culturing the resulting cells, preferably in the presence of uridine diphosphate galactose (UDP-Gal), uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) and/or cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac), resulting in influenza production cells, preferably MDCK cells or Vero cells, with extended glycan structures. In other embodiments expression of said enzymes is induced by genetically modifying erythrocyte precursor cells. The term “erythrocyte precursor cell” as used herein refers to any cell involved in the process of erythropoiesis, with the exclusion of erythrocytes. This term embraces hemocytoblasts (multipotent hematopoietic stem cells), multipotent stem cells, unipotent stem cells, pronormoblasts (also referred to as proerythroblasts or rubriblasts), basophilic or early normoblasts (also referred to as erythroblasts), polychromatic or intermediate normoblasts, orthochromatic or late normoblast, and reticulocytes. Erythrocytes derived from such genetically modified precursor cells are very suitable for methods and uses according to the present invention. Further provided is therefore a method for modifying cell surface glycans of erythrocyte precursor cells, wherein the method comprises the steps of: - providing erythrocyte precursor cells with nucleic acid molecules encoding a galactosyl transferase and an N-acetylglucosaminyl transferase that are capable of generating type II LacNAc structures; and - culturing the resulting cells, preferably in the presence of uridine diphosphate galactose (UDP-Gal) and/or uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), resulting in erythrocyte precursor cells with extended glycan structures. In some embodiments, a 2,6-sialyltransferase is added to the growing cells, either at the start of the culture or later, for instance 30 minutes, or 1 hour, or 2 hours, or 5 hours, or 10 hours, or 24 hours, or more than 24 hours, after the start of the culture. In some embodiments, a 2,6-sialyltransferase and cytidine-5'- monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac) are added to the growing cells. Alternatively, said cells are provided with a nucleic acid molecule encoding a 2,6-sialyltransferase. Accordingly, some embodiments provide a method for modifying cell surface glycans of erythrocyte precursor cells, wherein the method comprises the steps of: - providing erythrocyte precursor cells with nucleic acid molecules encoding a galactosyl transferase and an N-acetylglucosaminyl transferase that are capable of generating type II LacNAc structures; - providing said cells with a nucleic acid molecule encoding a 2,6- sialyltransferase; and - culturing the resulting cells, preferably in the presence of uridine diphosphate galactose (UDP-Gal), uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) and/or cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac), resulting in erythrocyte precursor cells with extended glycan structures. As stated herein before, in preferred embodiments said galactosyl transferase is galactosyltransferase B4GalT1. Said N-acetyl glucosaminyl transferase is preferably N-acetyl glucosaminyl transferase B3GNT2. Said 2,6- sialyl transferase is preferably ST6Gal1. Further provided is therefore a method for modifying cell surface glycans of glycan-containing cells, the method comprising the steps of: - providing glycan-containing cells with nucleic acid molecules encoding galactosyltransferase B4GalT1 and N-acetyl glucosaminyl transferase B3GNT2; and - culturing the resulting cells, preferably in the presence of uridine diphosphate galactose (UDP-Gal) and/or uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), resulting in cells with extended glycan structures. In some embodiments, ST6Gal1 is added to the growing cells, either at the start of the culture or later, for instance 30 minutes, or 1 hour, or 2 hours, or 5 hours, or 10 hours, or 24 hours, or more than 24 hours, after the start of the culture. In some embodiments, ST6Gal1 and cytidine-5'-monophosphate- N-acetylneuraminic acid (CMP-Neu5Ac) are added to the growing cells. Alternatively, said cells are provided with a nucleic acid molecule encoding ST6Gal1. Accordingly, some embodiments provide a method for modifying cell surface glycans of glycan-containing cells, wherein the method comprises the steps of: - providing glycan-containing cells with nucleic acid molecules encoding galactosyltransferase B4GalT1 and N-acetyl glucosaminyl transferase B3GNT2; - providing said cells with a nucleic acid molecule encoding ST6Gal1; and - culturing the resulting cells, preferably in the presence of uridine diphosphate galactose (UDP-Gal), uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) and/or cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac), resulting in glycan-containing cells with extended glycan structures. Further provided is a method for modifying cell surface glycans of influenza production cells, preferably MDCK cells or Vero cells, the method comprising the steps of: - providing influenza production cells, preferably MDCK cells or Vero cells, with nucleic acid molecules encoding galactosyltransferase B4GalT1 and N-acetyl glucosaminyl transferase B3GNT2; and - culturing the resulting cells, preferably in the presence of uridine diphosphate galactose (UDP-Gal) and/or uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), resulting in cells with extended glycan structures. In some embodiments, ST6Gal1 is added to the growing influenza production cells, either at the start of the culture or later, for instance 30 minutes, or 1 hour, or 2 hours, or 5 hours, or 10 hours, or 24 hours, or more than 24 hours, after the start of the culture. In some embodiments, ST6Gal1 and cytidine-5'- monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac) are added to the growing influenza production cells. Alternatively, said influenza production cells are provided with a nucleic acid molecule encoding ST6Gal1. Accordingly, some embodiments provide a method for modifying cell surface glycans of influenza production cells, preferably MDCK cells or Vero cells, wherein the method comprises the steps of: - providing influenza production cells, preferably MDCK cells or Vero cells, with nucleic acid molecules encoding galactosyltransferase B4GalT1 and N- acetyl glucosaminyl transferase B3GNT2; - providing said influenza production cells with a nucleic acid molecule encoding ST6Gal1; and - culturing the resulting cells, preferably in the presence of uridine diphosphate galactose (UDP-Gal), uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) and/or cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac), resulting in influenza production cells, preferably MDCK cells or Vero cells, with extended glycan structures. Further provided is a method for modifying cell surface glycans of erythrocyte precursor cells, the method comprising the steps of: - providing erythrocyte precursor cells with nucleic acid molecules encoding galactosyltransferase B4GalT1 and N-acetyl glucosaminyl transferase B3GNT2; and - culturing the resulting cells, preferably in the presence of uridine diphosphate galactose (UDP-Gal) and/or uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), resulting in erythrocyte precursor cells with extended glycan structures. In some embodiments, ST6Gal1 is added to the growing erythrocyte precursor cells, either at the start of the culture or later, for instance 30 minutes, or 1 hour, or 2 hours, or 5 hours, or 10 hours, or 24 hours, or more than 24 hours, after the start of the culture. In some embodiments, ST6Gal1 and cytidine-5'- monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac) are added to the growing erythrocyte precursor cells. Alternatively, said erythrocyte precursor cells are provided with a nucleic acid molecule encoding ST6Gal1. Accordingly, some embodiments provide a method for modifying cell surface glycans of erythrocyte precursor cells, wherein the method comprises the steps of: - providing erythrocyte precursor cells with nucleic acid molecules encoding galactosyltransferase B4GalT1 and N-acetyl glucosaminyl transferase B3GNT2; - providing said erythrocyte precursor cells with a nucleic acid molecule encoding ST6Gal1; and - culturing the resulting cells, preferably in the presence of uridine diphosphate galactose (UDP-Gal), uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) and/or cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac), resulting in erythrocyte precursor cells with extended glycan structures. A cell obtainable by a method according to the present invention is also provided herewith. In some preferred embodiments, said cell obtainable by a method according to the invention is an erythrocyte. In some embodiments, said cell obtainable by a method according to the invention is an erythrocyte precursor cell, preferably a reticulocyte. In some embodiments, said cell obtainable by a method according to the invention is an influenza production cell. Preferably, said influenza production cell is selected from the group consisting of a MDCK cell, a Vero cell, a PER.C6 cell, an AGE1.CR cell, a PBG.PK2.1 cell, an EB66 cell, a DuckCelT-T17 cell, and a QOR/2E11 cell. In particularly preferred embodiments, said cell is an erythrocyte or a MDCK cell or a Vero cell, more preferably an erythrocyte or a Vero cell. Most preferably, said cell is an erythrocyte. Said erythrocyte is preferably a fowl erythrocyte, as fowl erythrocytes are preferred for HA and HI assays in view of their larger size as compared to mammalian erythrocytes. Some embodiments provide a kit of parts that is suitable for modifying cell surface glycans, wherein said kit comprises the above mentioned neuraminidase, galactosyl transferase, N-acetyl glucosaminyl transferase and 2,6-sialyltransferase. Further provided is therefore a kit of parts, the kit comprising a neuraminidase, a galactosyl transferase and an N-acetyl glucosaminyl transferase that are capable of generating type II LacNAc structures, and a 2,6-sialyltransferase. Optionally, said kit further comprises uridine diphosphate galactose (UDP-Gal), uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), and/or cytidine-5'- monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac). In some embodiments, a kit of parts according to the invention further comprises alkaline phosphatase, because alkaline phosphatase shifts the equilibrium of the enzymatic reaction, thereby increasing the yield of N-glycans with extended LacNAc structures. In order to be particularly useful for modifying cell surface glycans of living cells, a kit of parts according to the invention in some embodiments also comprises a cell culture medium or a constituent thereof, such as for instance Minimum Essential Medium (MEM), Dulbecco’s Modified Eagle’s Medium (DMEM), Eagle’s Minimum Essential Medium (EMEM), Roswell Park Memorial Institue (RPMI)- 1640 Medium or Iscove’s Modified Dulbecco’s Medium (IMDM), or a constituent thereof. In a preferred embodiment said cell culture medium is DMEM. In some embodiments, a kit of part according to the invention comprises an Arthrobacter Ureafaciens neuraminidase, galactosyltransferase B4GalT1, N-acetyl glucosaminyl transferase B3GNT2 and 2,6-sialyl transferase ST6Gal1. Further provided is therefore a kit of parts, comprising: - an Arthrobacter Ureafaciens neuraminidase; - galactosyltransferase B4GalT1; - N-acetyl glucosaminyl transferase B3GNT2; and - ST6Gal1. As described above, the presence of alkaline phosphatase is preferred because alkaline phosphatase shifts the equilibrium of the enzymatic reaction, thereby increasing the yield of N-glycans with extended LacNAc structures. Also provided is therefore a kit of parts, comprising: - an Arthrobacter Ureafaciens neuraminidase; - galactosyltransferase B4GalT1; - N-acetyl glucosaminyl transferase B3GNT2; - ST6Gal1; and - alkaline phosphatase. As described above, the presence of a cell culture medium or a constituent thereof renders a kit of parts particularly suitable for modifying cell surface glycans of living cells. Further provided is therefore a kit of parts, comprising: - an Arthrobacter Ureafaciens neuraminidase; - galactosyltransferase B4GalT1; - N-acetyl glucosaminyl transferase B3GNT2; - ST6Gal1; and - a cell culture medium or a constituent thereof, such as for instance Minimum Essential Medium (MEM), Dulbecco’s Modified Eagle’s Medium (DMEM), Eagle’s Minimum Essential Medium (EMEM), Roswell Park Memorial Institue (RPMI)- 1640 Medium or Iscove’s Modified Dulbecco’s Medium (IMDM) or a constituent thereof, preferably DMEM. Some embodiments provide a method for modifying cell surface glycans, comprising the steps of: - incubating cells with a neuraminidase in order to desialylate cell surface glycans of said cells; and - coupling a minimal receptor according to the invention to the resulting cells. In some embodiments, said minimal receptor according to the invention is coupled to the resulting cells using an enzyme like ST6Gal1. In some embodiments, said minimal receptor according to the invention is first attached to a CMP-Neu5Ac, where after the resulting construct is coupled to the resulting cells using ST6Gal1. Further provided is a kit of parts that is suitable for said method. Some embodiments therefore comprise a kit of parts, comprising: - a neuraminidase; and - a multi-antennary glycan, wherein at least one arm of said multi-antennary glycan comprises at least three LacNAc moieties. Also provided is a kit of parts, comprising: - a neuraminidase; - a multi-antennary glycan, wherein at least one arm of said multi-antennary glycan comprises at least three LacNAc moieties; and - a 2,6-sialyltransferase. As described above, a kit of parts according to the invention is very suitable for modifying cell surface glycans present on cells. In some embodiments, a kit of parts is therefore provided comprising; - a neuraminidase; - a multi-antennary glycan, wherein at least one arm of said multi-antennary glycan comprises at least three LacNAc moieties; - a 2,6-sialyltransferase; and - a cell culture medium or a constituent thereof. In a preferred embodiment, a kit of parts is provided comprising; - a neuraminidase; - a multi-antennary glycan, wherein at least one arm of said multi-antennary glycan comprises at least three LacNAc moieties; - a 2,6-sialyltransferase; and - alkaline phosphatase. In some embodiments, said 2,6-sialyltransferase is ST6Gal1. In some embodiments said multi-antennary glycan is a multi-antennary glycan wherein a first arm of said multi-antennary glycan comprises at least one LacNAc moiety and wherein a second arm of said multi-antennary glycan comprises at least three LacNAc moieties. Cells and compounds according to the invention are suitable for detecting and/or characterizing influenza viruses in various ways. In some embodiments, erythrocytes according to the invention are used in an HA assay or an HI assay. As shown in the Examples, recently circulating A/H3N2 viruses can now be characterized with the use of erythrocytes according to the present invention, whereas prior art HA assays and HI assays failed to detect these recently circulating A/H3N2 viruses. Some embodiments therefore provide a use of an erythrocyte according to the invention for performing a hemagglutination assay or a hemagglutination inhibition assay. Also provided is method for performing a hemagglutination assay, comprising the step of incubating erythrocytes with influenza virus and subsequently determining whether said erythrocytes are agglutinated, characterized in that erythrocytes according to the invention are used. Also provided is a method for performing a hemagglutination inhibition assay, comprising the step of incubating influenza virus with antibodies that are specific for an influenza HA protein, and with erythrocytes, and subsequently determining whether agglutination of erythrocytes is counteracted by said antibodies, characterized in that erythrocytes according to the invention are used. Also provided is a method for detecting, identifying and/or characterizing an influenza virus, comprising performing a hemagglutination inhibition assay with said influenza virus, with antibodies that are specific for an influenza HA protein, and with erythrocytes, characterized in that erythrocytes according to the invention are used. In some embodiments, compounds according to the invention, preferably particles or beads according to the invention, are used in an agglutination assay or in an agglutination inhibition assay. General protocols for using beads in agglutination assays and agglutination inhibition assays are known in the art. For instance, Xu et al. (2005) describe an agglutination assay based on polystyrene beads sensitized with inactivated avian influenza virus H5N1 particles. Non- limiting examples of suitable particles or beads for use according to the invention are particles or beads that comprise latex, polystyrene, polyacrylamide, gold, glass, metal, natural polymer, synthetic polymer, polylactic acid or silica. Some embodiments therefore provide a use of a compound according to the invention, preferably a particle or bead according to the invention, for performing an agglutination assay or an agglutination inhibition assay. Also provided is method for performing an agglutination assay, comprising the step of incubating compounds, preferably particles or beads, with influenza virus and subsequently determining whether said compounds, particles or beads are agglutinated, characterized in that compounds, preferably particles or beads, according to the invention are used. Also provided is a method for performing an agglutination inhibition assay, comprising the step of incubating influenza virus with antibodies that are specific for an influenza HA protein, and with compounds, preferably particles or beads, and subsequently determining whether agglutination of compounds, particles or beads is counteracted by said antibodies, characterized in that compounds, preferably particles or beads, according to the present invention are used. Also provided is a method for detecting, identifying and/or characterizing an influenza virus, comprising performing an agglutination assay with said influenza virus and with compounds, particles or beads, characterized in that compounds, preferably particles or beads, according to the invention are used. Some embodiments provide a kit of parts for a HA assay or an HI assay, wherein said kit comprises erythrocytes according to the present invention. In some embodiments said kit of parts further comprises a solid support, such as for instance a 96-well microtiter plate, useful for incubating erythrocytes with influenza virus (or with influenza HA proteins or with influenza HA antigens). Further provided is therefore a kit of parts for a hemagglutination assay or for a hemagglutination inhibition assay, the kit comprising erythrocytes and a solid support, preferably a microtiter plate, wherein said erythrocytes comprise erythrocytes according to the present invention. Some embodiments provide a kit of parts for a hemagglutination inhibition assay, comprising erythrocytes according to the present invention and antibodies that are specific for the HA proteins of one or more influenza strains. Further provided is therefore a kit of parts for a hemagglutination inhibition assay, the kit comprising erythrocytes and antibodies that are specific for an influenza HA protein, wherein said erythrocytes comprise erythrocytes according to the present invention. Some embodiments provide a kit of parts for an agglutination assay or for an agglutination inhibition assay, wherein said kit comprises compounds according to the present invention, preferably particles or beads according to the present invention. In some embodiments said kit of parts further comprises a solid support, such as for instance a 96-well microtiter plate, useful for incubating compounds, particles or beads with influenza virus (or with influenza HA proteins or with influenza HA antigens). Further provided is therefore a kit of parts for an agglutination assay or for an agglutination inhibition assay, the kit comprising compounds, preferably particles or beads, and a solid support, preferably a microtiter plate, wherein said compounds, particles or beads comprise compounds, particles or beads according to the present invention. Some embodiments provide a kit of parts for an agglutination inhibition assay, comprising compounds, preferably particles or beads, according to the present invention and antibodies that are specific for the HA proteins of one or more influenza strains. Further provided is therefore a kit of parts for an agglutination inhibition assay, the kit comprising compounds, preferably particles or beads, and antibodies that are specific for an influenza HA protein, wherein said compounds, particles or beads comprise compounds, particles or beads according to the present invention. In some embodiments, a cell or compound according to the invention is used in a binding assay. For instance, if an individual is suspected of suffering from an influenza virus infection, a virus-containing sample, such as for instance a saliva, sputum, blood or tissue sample, can be obtained from said individual. Subsequently, said sample can be tested for the presence of influenza virus using a cell or compound according to the invention. In some embodiments, cells or compounds according to the invention are incubated with said sample, or with a virus-containing portion thereof, allowing influenza virus to bind said cells or compounds according to the invention. Bound virus, if present, can be isolated and/or detected using any method known in the art, for instance using magnetic beads and/or antibodies against the stem region of the HA protein. In some embodiments, an ELISA assay is used with a solid surface that comprises a multi- antennary glycan, wherein at least one arm of said multi-antennary glycan comprises at least three LacNAc moieties, preferably at least three consecutive LacNAc moieties. In some preferred embodiments, said solid surface comprises a multi-antennary glycan, wherein a first arm of said multi-antennary glycan comprises at least one LacNAc moiety and wherein a second arm of said multi- antennary glycan comprises at least three LacNAc moieties, preferably at least three consecutive LacNAc moieties. In some embodiments, influenza virus is detected by flow cytometry, using compounds, preferably particles or beads, comprising a multi-antennary glycan, wherein at least one arm of said multi- antennary glycan comprises at least three LacNAc moieties, preferably at least three consecutive LacNAc moieties. In some preferred embodiments, said compounds or beads comprise a multi-antennary glycan, wherein a first arm of said multi-antennary glycan comprises at least one LacNAc moiety and wherein a second arm of said multi-antennary glycan comprises at least three LacNAc moieties, preferably at least three consecutive LacNAc moieties. In the above mentioned assays, the use of non-cellular compounds according to the invention is preferred, since non-cellular compounds are typically more suitable for handling as compared to cells. In some embodiments, a cell or compound according to the invention is labeled, for instance fluorescently labeled or radioactively labeled. In some embodiments, such labeled cell or compound according to the invention is incubated with a sample, or with a virus-containing part thereof, such that virus is allowed to bind. Subsequently, cells or compounds according to the invention that are bound to influenza virus are collected, while unbound cells or compounds are preferably washed away. Bound cells or compounds can then be detected via their label. The invention thus further provides a use of a cell or compound according to the invention for detection, identification and/or characterization of an influenza virus. Also provided is a cell or compound according to the invention for use in diagnosis of an influenza virus infection. In some embodiments, said influenza virus is an influenza A virus, preferably a H3N2 influenza A virus. Some embodiments provide a method for determining whether an influenza virus is present in a sample, the method comprising: - contacting said sample, or a virus-containing part thereof, with a cell or compound according to the invention, - allowing said cell or compound to bind influenza virus, if present, and - determining whether influenza virus is bound to said cell or compound, thereby determining whether an influenza virus is present. Preferably, the concentration of said influenza virus in said sample is determined. In some embodiments, said influenza virus is an influenza A virus, preferably a H3N2 influenza A virus. Further provided is a method for determining whether an individual is suffering from an influenza virus infection, the method comprising: - contacting a sample from said individual, or a virus-containing part thereof, with a cell or compound according to the invention, - allowing said cell or compound to bind influenza virus, if present, and - determining whether influenza virus is bound to said cell or compound, thereby determining whether said individual is suffering from an influenza virus infection. Preferably said individual is a human individual. In some embodiments, said individual is a non-human animal. In some embodiments, said influenza virus is an influenza A virus, preferably a H3N2 influenza A virus. In some embodiments, a method according to the present invention is used for clade testing. This is for instance useful for vaccine development. Now that a minimal receptor according to the invention has been provided, more recently evolved influenza strains can be characterized, for instance in an HI assay using reference antisera against influenza viruses of known clades, in order to determine to which clade a certain test virus belongs. Further provided is therefore a use of a cell or compound according to the invention for clade testing of an influenza virus. In some preferred embodiments, said cell is an erythrocyte. In some embodiments, said influenza virus is an influenza A virus, preferably a H3N2 influenza A virus. The invention further pertains to novel glycan arrays comprising bi- antennary and/or multi-antennary N-glycans. Existing prior art glycan microarrays appeared to be unsuitable for identifying a minimal receptor according to the present invention. For instance, Blixt et al (2004) disclose a glycan array that mainly consists of linear glycans. A few bi-antennary and multi-antennary N-glycans are also present on the glycan array of Blixt et al, but these glycans either contain branches consisting of mannose moieties, without LacNAc moieties, or they contain branches having at most one LacNAc moiety. Of note, the glycan array of Blixt et al does not contain a minimal receptor according to the present invention. The present inventors have constructed a novel glycan array that is populated with multi-antennary N-glycans having different numbers of LacNAc repeating units. This array is for instance useful for identifying a minimal receptor according to the invention and for identifying binding specificities of recently circulating A/H3N2 viruses of clades 3C.1, 3C.2a1, 3C.2a1b, 3C.2a, 3C.2a2, 3C.3 or 3C.3a, such as A/NL/761/09, A/NL/2413/16, A/NL/751/17, A/NL/1797/17, A/NL/295/19, A/NL/314/19, A/NL/371/19, A/NL/03466/17, NIB-112 (A/Switzerland/8060/17), A/NL/10616/18, A/NL/622/12 and/or A/Switzerland/9715293/13. A glycan array according to the present invention comprises a plurality of N-glycans, wherein at least one N-glycan comprises a bi- antennary or multi-antennary glycan wherein at least one arm of said bi- antennary or multi-antennary glycan comprises at least three LacNAc moieties. Further provided is therefore a glycan array comprising a solid support, such as for instance a plate, that comprises at least 5 different N-glycans, wherein at least one N-glycan comprises a bi-antennary or multi-antennary N-glycan wherein at least one arm of said bi-antennary or multi-antennary N-glycan comprises at least three LacNAc moieties. In some preferred embodiments, said array comprises at least one bi-antennary or multi-antennary N-glycan wherein a first arm of said bi- antennary or multi-antennary N-glycan comprises at least one LacNAc moiety and wherein a second arm of said bi-antennary or multi-antennary N-glycan comprises at least three LacNAc moieties. Further provided is therefore a glycan array comprising a solid support, such as for instance a plate, that comprises at least 5 different N-glycans, wherein at least one N-glycan comprises a bi-antennary or multi-antennary glycan wherein a first arm of said bi-antennary or multi- antennary N-glycan comprises at least one LacNAc moiety and wherein a second arm of said bi-antennary or multi-antennary glycan comprises at least three LacNAc moieties. In some embodiments, said second arm of said bi-antennary or multi-antennary glycan comprises at least three consecutive LacNAc moieties. In preferred embodiments said bi-antennary or multi-antennary N-glycan has an asymmetric structure, meaning that at least two arms of said bi-antennary or multi-antennary N-glycan have a different number of LacNAc moieties. As used herein, a bi-antennary or multi-antennary N-glycan having an asymmetric structure is also referred to as an “asymmetric bi-antennary or multi-antennary N-glycan”, or an “asymmetric N-glycan”. A non-limiting example of an asymmetric bi-antennary or multi-antennary N-glycan is an N-glycan that comprises a first arm wherein the number of LacNAc moieties is three, and a second arm wherein the number of LacNAc moieties is one. Another non-limiting example of an asymmetric bi-antennary or multi-antennary N-glycan is an N-glycan that comprises a first arm wherein the number of LacNAc moieties is three or four, and a second arm wherein the number of LacNAc moieties is two. Hence, the number of LacNAc moieties in said first and second arms of an asymmetric N-glycan is different. Besides the recited numbers of LacNAc moieties, said first and second arms may comprise other non-LacNAc moieties, such as for instance one or more galactose, N-acetylglucosamine (GlcNAc), fucose and/or mannose moieties. The use of glycan arrays with N-glycans that have asymmetric structures is preferred because such glycan arrays more closely resemble the natural glycomes of cells. Hence, glycan arrays with asymmetric N-glycans are particularly suitable for the detection, identification and/or characterization of an influenza virus. In some embodiments, a glycan array according to the invention comprises at least 5 or at least 10 or at least 20 different bi-antennary and/or multi- antennary N-glycans, wherein at least one arm of said at least 5 or at least 10 or at least 20 different bi-antennary or multi-antennary N-glycans comprises at least three LacNAc moieties. In some embodiments, a glycan array according to the invention comprises at least 5 or at least 10 or at least 20 different bi-antennary and/or multi- antennary N-glycans, wherein a first arm of said at least 5 or at least 10 or at least 20 different bi-antennary or multi-antennary N-glycans comprises at least one LacNAc moiety and wherein a second arm of said at least 5 or at least 10 or at least 20 different bi-antennary or multi-antennary N-glycans comprises at least three LacNAc moieties. Said at least 5 or at least 10 or at least 20 different bi-antennary or multi-antennary N-glycans preferably have asymmetric structures. In some embodiments, a glycan array according to the invention comprises at least 30 or at least 40 different bi-antennary and/or multi-antennary N-glycans, wherein at least one arm of said at least 30 or at least 40 different bi-antennary or multi-antennary N-glycans comprises at least three LacNAc moieties. In some embodiments, said glycan array comprises at least 30 or at least 40 different bi- antennary and/or multi-antennary N-glycans, wherein a first arm of said at least 30 or at least 40 different bi-antennary or multi-antennary N-glycans comprises at least one LacNAc moiety and wherein a second arm of said at least 30 or at least 40 different bi-antennary or multi-antennary N-glycans comprises at least three LacNAc moieties. Said at least 30 or at least 40 different bi-antennary or multi- antennary N-glycans preferably have asymmetric structures. In some embodiments, a glycan array according to the invention comprises at least 50 or at least 100 different bi-antennary and/or multi-antennary N-glycans, wherein at least one arm of said at least 50 or at least 100 different bi-antennary or multi-antennary N-glycans comprises at least three LacNAc moieties. In some preferred embodiments, said glycan array comprises at least 50 or at least 100 different bi-antennary and/or multi-antennary N-glycans, wherein a first arm of said at least 50 or at least 100 different bi-antennary or multi-antennary N-glycans comprises at least one LacNAc moiety and wherein a second arm of said at least 50 or at least 100 different bi-antennary or multi-antennary N-glycans comprises at least three LacNAc moieties. Said at least 50 or at least 100 different bi-antennary or multi-antennary N-glycans preferably have asymmetric structures. Further provided is a glycan array, characterized in that at least 50% of the glycans of said array are bi-antennary or multi-antennary N-glycans. In some embodiments, at least 60% of the glycans of said array are bi-antennary or multi- antennary N-glycans. Preferably, at least 70% or at least 80% of the glycans of said array are bi-antennary or multi-antennary N-glycans. More preferably, at least 90% or at least 95% of the glycans of said array are bi-antennary or multi- antennary N-glycans. Such glycan array according to the invention wherein at least 50% of the glycans of said array are bi-antennary or multi-antennary N-glycans preferably comprises N-glycans with LacNAc moieties. More preferably, at least 50% of the glycans of said array are bi-antennary or multi-antennary N-glycans wherein at least one arm comprises at least three LacNAc moieties. In some preferred embodiments, at least 50% of the glycans of said array are N-glycans wherein a first arm of said bi-antennary or multi-antennary glycans comprises at least one LacNAc moiety and wherein a second arm of said bi- antennary or multi-antennary glycans comprises at least three LacNAc moieties. In some embodiments, said second arm of said bi-antennary or multi-antennary glycan comprises at least three consecutive LacNAc moieties. Preferred embodiments provide a glycan array according to the present invention, wherein at least 30% of the glycans of said array are bi-antennary or multi-antennary N-glycans having an asymmetric structure. In some embodiments, at least 40% of the glycans of an array according to the invention are bi-antennary or multi-antennary N-glycans having an asymmetric structure. Preferably, at least 50% or at least 60% of the glycans of an array according to the invention are bi-antennary or multi-antennary N-glycans having an asymmetric structure. More preferably, at least 70% or at least 80% of the glycans of an array according to the invention are bi-antennary or multi-antennary N-glycans having an asymmetric structure. Most preferably, at least 90% of the glycans of an array according to the invention are bi-antennary or multi-antennary N-glycans having an asymmetric structure. Said asymmetric bi-antennary or multi-antennary glycans preferably comprise a first arm comprising at least three LacNAc moieties and a second arm comprising at least one LacNAc moiety. A glycan array according to the present invention preferably comprises compounds according to the invention, having at their surface at least one minimal receptor according to the invention. In some embodiments, at least 30% or at least 40% of the compounds of a glycan array according to the invention are compounds according to the invention, having at their surface at least one minimal receptor according to the invention. Preferably, at least 50% or at least 60% of the compounds of a glycan array according to the invention are compounds according to the invention, having at their surface at least one minimal receptor according to the invention. More preferably, at least 70% or at least 80% of the compounds of a glycan array according to the invention are compounds according to the invention, having at their surface at least one minimal receptor according to the invention. Most preferably, at least 90% or at least 95% of the compounds of a glycan array according to the invention are compounds according to the invention, having at their surface at least one minimal receptor according to the invention. Methods for coupling glycans to a solid support are known in the art. For instance, as a non-limiting example, glycans are printed on amine reactive, NHS activated glass slides, as described in the current Examples and in Blixt et al. Alternatively, other coupling methods may be applied. The novel glycan arrays according to the invention are particularly suitable for investigating ligand requirements of glycan binding proteins such as, but not limited to, influenza HA proteins, lectins and antibodies. In some embodiments a glycan array according to the invention is incubated with a candidate glycan binding compound or virus in order to determine whether glycans are bound and, if so, which types of glycans are bound. This way, the ligand requirements of the candidate compound or virus can be assessed. Further provided is therefore a use of a glycan array according to the invention for determining whether a test compound or test virus is able to bind a glycan. Also provided is a use of a glycan array according to the invention for identifying a glycan that is able to be bound by a test compound or test virus. In some embodiments, said test virus is an influenza virus. In some embodiments, said test virus is an influenza A virus, preferably an A/H3N2 virus. Also provided is a use of a glycan array according to the invention for characterizing the glycan ligand requirement of a test compound or test virus. In some embodiments, said test virus is an influenza virus. In some embodiments, said test virus is an influenza A virus, preferably an A/H3N2 virus. Some embodiments provide a method for determining whether a test compound or test virus is able to bind a glycan, the method comprising incubating a glycan array according to the invention with a test compound or test virus, and determining whether said test compound or test virus has bound one or more glycans of said array. In some embodiments, said method further comprises determining which glycan or glycans is/are bound by said test compound or test virus, thereby characterizing the ligand requirement of said test compound or test virus. Further provided is therefore a method for characterizing the glycan ligand requirement of a test compound or test virus, the method comprising incubating a glycan array according to the invention with a test compound or test virus, allowing said test compound or test virus to bind one or more glycans of said array, and determining which glycan or glycans is/are bound by said test compound or test virus, thereby characterizing the glycan ligand requirement of said test compound or test virus. In some embodiments, said test virus is an influenza virus. In some embodiments, said test virus is an influenza A virus, preferably an A/H3N2 virus. In some embodiments, a glycan array according to the present invention is used in a binding assay. For instance, if an individual is suspected of suffering from an influenza virus infection, a virus-containing sample, such as for instance a saliva, sputum, blood or tissue sample, can be obtained from said individual. Subsequently, said sample can be tested for the presence of influenza virus using a glycan array according to the invention. In some embodiments, a glycan array according to the invention is incubated with said sample, or with a virus-containing portion thereof, allowing influenza virus to bind said glycan array according to the invention. Bound virus, if present, can be isolated and/or detected using any method known in the art, for instance using antibodies against the stem region of the HA protein. In some embodiments, an ELISA assay is used. The invention thus further provides a use of a glycan array according to the invention for detection, identification and/or characterization of an influenza virus. Also provided is a glycan array according to the invention for use in diagnosis of an influenza virus infection. In some embodiments, said influenza virus is an influenza A virus, preferably a H3N2 influenza A virus. Some embodiments provide a method for determining whether an influenza virus is present in a sample, the method comprising: - contacting said sample, or a virus-containing part thereof, with a glycan array according to the invention, - allowing said glycan array to bind influenza virus, if present, and - determining whether influenza virus is bound to said glycan array, thereby determining whether an influenza virus is present. In some embodiments, said influenza virus is an influenza A virus, preferably a H3N2 influenza A virus. Further provided is a method for determining whether an individual is suffering from an influenza virus infection, the method comprising: - contacting a sample from said individual, or a virus-containing part thereof, with a glycan array according to the invention, - allowing said glycan array to bind influenza virus, if present, and - determining whether influenza virus is bound to said glycan array, thereby determining whether said individual is suffering from an influenza virus infection. Preferably said individual is a human individual. In some embodiments, said individual is a non-human animal. In some embodiments, said influenza virus is an influenza A virus, preferably a H3N2 influenza A virus. While the current application may describe features as part of the same embodiment or as parts of separate embodiments, the scope of the present invention also includes embodiments comprising any combination of all or some of the features described herein. The invention is further explained in the following examples. These examples do not limit the scope of the invention, but merely serve to clarify the invention.

Brief description of the drawings Figure 1. Synthesis of asymmetric, biantennary N-glycans. Figure 2. Binding specificity of control lectins ECA, MAL1 and SNA and CR8020 A/H3N2 stem antibody. Glycans were printed on NHS glass slides and receptor specificity of the biotinylated lectins and the CR8020 antibody was visualized using Streptavidin-AlexaFluor635. The mean signal was plotted in relative fluorescence units (RFU) and standard deviations were calculated for each glycan printed in replicates of 6. Glycans #1-#3: non-sialylated; #4-#6: α2,3-linked Neu5Ac; #7-#17: α2,6-linked Neu5Ac. Figure 3. Stability assay of glyco-engineered erythrocytes. Shown are the HA titers of A/H3N2 A/NL/761/09 and A/H3N2 A/NL/816/91 using unmodified (left) or glyco-engineered (right) erythrocytes from chicken (red) and turkey (blue). Glyco-engineering and HA assays were performed in full biological triplicates and means are plotted ± SEM. Figure 4. Concentration dependent response of representative A/H3N2 viruses in glycan microarray analysis. Glycans were printed on NHS glass slides and receptor specificity was visualized using the human CR8020 antibody followed with a goat anti-human alexa555 antibody. Viruses were applied to the assay 2x and 4x diluted from the original isolate. The mean signal was plotted in relative fluorescence units (RFU) and standard deviation was calculated for each glycan printed in replicates of 6. Glycans #1-#3: non-sialylated; #4-#6: α2,3-linked Neu5Ac; #7-#17 and A-C: α2,6-linked Neu5Ac. Figure 5. (A) Schematic overview of enzymatic modification of erythrocytes. (B) Glycomic analysis of N-glycosylation of enzymatically modified chicken and turkey erythrocytes. The 30 most abundant N-glycans on the enzymatically modified erythrocytes from chicken and turkey (based on relative intensity, excluding high-mannose type N-glycans, for all structures refer to SI Data S1) sorted by abundance and number of LacNAc units. Proposed structures are assigned to detected glycan compositions. (C) Hemagglutination assays with representative A/H3N2 viruses and modified erythrocytes. NL91, NL03, NL09, NL17 and NL19 tested with modified erythrocytes (2,6-Sia Poly-LN) from chicken (blue) and turkey (red). Unmodified, 2,6 resialylated (2,6- Sia) and desialylated (Poly-LN) erythrocytes were added as controls. Assays were done in full biological triplicates in the presence of 20 nM Oseltamivir and the means ± SEM were plotted. Figure 6. Glycomic analysis of N-glycosylation of turkey erythrocytes released by Endo F2. The data is sorted by abundance and number of LacNAc units. Proposed structures are assigned to detected glycan compositions. Figure 7. (A) Receptor binding specificities of representative H3N2 viruses using glycan microarray analysis. Glycans were printed on NHS glass slides and receptor specificity was visualized using the human CR8020 antibody followed with a goat anti-human alexa555 antibody. The mean signal was plotted in relative fluorescence units (RFU) and standard deviation was calculated for each glycan printed in replicates of 6. The data are representative of three independent assays. Glycans #1-#3: non-sialylated; #4-#6: α2,3-linked Neu5Ac; #7-#17: α2,6- linked Neu5Ac. (B) Glycomic analysis of N-glycosylation of chicken and turkey erythrocytes. The 30 most abundant N-glycans on erythrocytes from chicken and turkey (based on relative intensity, excluding high-mannose type N- glycans) sorted by abundance and number of LacNAc units. Proposed structures are assigned to detected glycan compositions. Figure 8. Relative cell surface occupation of complex and high mannose glycans on chicken and turkey erythrocytes. The percentual distribution of complex and high mannose glycans was calculated based on the data from the glycomic analysis of untreated erythrocytes. Figure 9. Correlation of Hemagglutination inhibition and Focus reduction titers by recent A/H3N2 IAV. Shown is the correlation between every HI titer and FRA titer for all viruses and sera as depicted in Table 1 and Table 3. The titers of the focus reduction assay duplicates were averaged.

Examples Materials and Methods Synthesis of biantennary N-glycans Enzymes were expressed as previously described (B3GNT2 and B4GalT1 in (1), PmST1 M144L P34H in (2)). The reaction mixtures were purified using a size exclusion Biogel (P2) from BioRad in Econo glass columns (0.7 x 30 cm / 1.5 x 30 cm / 1.5 x 50 cm) coupled to a BioFrac fraction collector (BioRad). Carbohydrate- containing fractions were detected by thin layer chromatography and an appropriate staining reagent (15 mL AcOH and 3.5 mL p-Anisaldehyde in 350 mL EtOH and 50 mL H ₂ SO ₄ ). Reagents were purchased from Sigma-Aldrich. Uridine 5’-diphosphogalactose (UDP-Gal), uridine 5’-diphospho-N-acetyl-glucosamine (UDP-GlcNAc) and cytidine-5’-monophospho-N-acetylneuraminic acid (CMP- Neu5Ac) were obtained from Roche Diagnostics [UDP-Gal: Cat# 07703562103; UDP-GlcNAc: Cat# 06369855103; CMP-Neu5Ac: Cat# 05974003103]. Final products were purified by high performance liquid chromatography (HPLC) using an XBridge HILIC column (10 mm (∅) x 250 mm (l), 5 µm particle size) on a semi- preparative liquid chromatography system from Shimadzu (LC-20AT, SIL-20A, CBM-20A, SPD-20A, FRC-10A). The purification was done using 10 mM NH ₄ HCOO in 10% H ₂ O in MeCN (buffer B) and 10 mM NH ₄ HCOO in 100% H ₂ O (buffer A). The progress of the reactions was monitored on a liquid chromatography mass spectrometry system (LCMS) from Shimadzu (system controller: SCL10A-VP; HPLC pumps: LC10AD-VP; injector: SIL10AD-VP) using a ZIC HILIC column (ZeQuant, PEEK coated guard HPLC column, 3.5 µm particle size, 20 x 2.1 mm). The LC system was attached to a Bruker Daltonics microTOF-Q mass spectrometer. Synthesis of N-glycans Sialylglycopeptide (SGP) was extracted from egg yolk and further enzymatically modified to yield compound 1, which was used as a starting material for the synthesis (3,4). As depicted in Figure 1, the terminal galactose of compound 1 was sialylated with an α2,6 specific sialyltransferase mutant P34H/M144L from Pasteurella multocida and CMP-Neu5Ac providing compound 2 (2). This compound was subsequently extended with N-acetyllactosamine (LacNAc) repeats by using mammalian β1,4-galactosyltransferase 1 (B4GalT1) and β1,3-N-acetylglucosamine transferase (B3GNT2) with their corresponding nucleotide sugars UDP-Gal and UDP-GlcNAc, respectively. As a result, compound 3, 4 and 6 with one, two and three consecutive LacNAc repeats on the MGAT1 (Mannose-3) branch and a terminal α2,6 linked N-acetylneuraminic acid (Neu5Ac) on the MGAT2 (Mannose- 6) branch were obtained. These intermediates were used to synthesize the bisialylated compounds 5 and 7 by sialylating the extended MGAT1 (Mannose-3) branch with the α2,6 specific sialyltransferase mutant from Pasteurella multocida and CMP-Neu5Ac. Compounds 9 and 11, modified a with single terminal Neu5Ac on either branch, were prepared by first quantitatively desialylating the intermediates 4 and 6 in an aqueous solution of acetic acid. Afterwards, sialyltransferase ST6Gal1 and CMP-Neu5Ac was used to install a single terminal Neu5Ac moiety providing the products 9 and 11. General procedure for the installation of α2,6-linked Neu5Ac using PmST1 (P34H/M144L) (2): The acceptor and CMP-Neu5Ac (2.5 eq) were dissolved in a Tris buffer (100 mM, pH 9, 0.1 wt% BSA) to obtain a concentration of 5 mM. PmST1 M144L P34H (42 µg per µmol acceptor) and CIAP (1 u µL ^-1 , 1 u per µmol of added nucleotide) were added to the reaction mixture and it was incubated overnight at 37 °C with gentle shaking. The progress of the reaction was monitored by LCMS. In case of incomplete conversion after 18 h, additional PmST1 M144L P34H (20 µg per µmol acceptor) was added and the reaction mixture incubated at 37 °C for an additional 24 h. After completion, the reaction mixture was lyophilized and applied to size exclusion chromatography. Carbohydrate-containing fractions were purified by HPLC (2: 68%B-66%B in 60 min, 3.3 mL min ^-1 ; 5: 65%B-64%B in 60 min, 3.3 mL min ^-1 ; 7: 65%B-50%B in 100 min, 3.3 mL min ^-1 ) providing the product as a white powder (2: 16.9 mg, 82%, 5: 0.48 mg, 17%; 7: 1.7 mg, 46%). General procedure for the installation of β1,3-linked glucose using B3GNT2: The acceptor and UDP-GlcNAc (1.5 eq) were dissolved in a HEPES buffer (50 mM, pH 9.6, 0.1 wt% BSA) containing DTT (1 mM) and MnCl ₂ (20 mM) to obtain a concentration of 5 mM. B3GnT2 (30 µg per µmol acceptor) and CIAP (1 u µL ^-1 , 1 u per µmol of added nucleotide) were added to the reaction mixture and it was incubated overnight at 37 °C with gentle shaking. The progress of the reaction was monitored by LCMS. In case of incomplete conversion after 18 h, additional UDP- GlcNAc (0.5 eq), CIAP (1 u µL ^-1 , 1 u per µmol of added nucleotide) and B3GNT2 (15 µg per µmol acceptor) were added and the reaction mixture incubated at 37 °C for an additional 24 h. After completion the reaction mixture was lyophilized and applied to size exclusion chromatography. Carbohydrate-containing fractions were lyophilized and used without further purification. General procedure for the installation of β1,4-linked galactose using B4GalT1: The acceptor and UDP-Gal (1.5 eq) were dissolved in a Tris buffer (50 mM, pH 7.3, 0.1wt% BSA) containing MnCl ₂ (20 mM) to obtain a concentration of 5 mM. B4GalT1 (20 µg per µmol acceptor) and CIAP (1 u µL ^-1 , 1 u per µmol of added nucleotide) were added to the reaction mixture and it was incubated overnight at 37 °C with gentle shaking. The progress of the reaction was monitored by LCMS. In case of incomplete conversion after 18 h, additional UDP-Gal (0.5 eq), CIAP (1 u µL ^-1 , 1 u per µmol of added nucleotide) and B4GalT1 (10 µg per µmol acceptor) were added and the reaction mixture incubated at 37 °C for 24 h. After completion the reaction mixture was lyophilized and applied to size exclusion chromatography. Carbohydrate-containing fractions were purified by HPLC (3: 68%B-65%B in 60 min, 3.4 mL min ^-1 ; 4: 65%B for 60 min, 3.4 mL min ^-1 ; 6: 65%B for 60 min, 3.3 mL min ^-1 ; ) providing the products as white powders (3: 16.4 mg, 88%; 4: 19.2 mg, 89%; 6: 11.3 mg, 75%). General procedure for the removal terminal Neu5Ac: The substrate was dissolved in an aqueous solution of acetic acid (2 M) and kept at 65 °C for 24 h. The solvent was removed in an N2 flow and the reaction mixture was applied to size exclusion chromatography. Carbohydrate-containing fractions were lyophilized and used without further purification. General procedure for the installation of α2,6-linked Neu5Ac using ST6Gal1: The acceptor and CMP-Neu5Ac (1.1 eq) were dissolved in a Tris buffer (50 mM, pH 7.3, 0.1 wt% BSA) to obtain a concentration of 2 mM. ST6Gal1 (42 µg per µmol acceptor) was added to the reaction mixture and it was incubated overnight at 37 °C with gentle shaking. The progress of the reaction was monitored by LCMS. In case of incomplete conversion after 18 h, additional ST6Gal1 (20 µg per µmol acceptor) was added and the reaction mixture incubated at 37 °C for 24 h. After completion the reaction mixture was lyophilized and applied to size exclusion chromatography. Carbohydrate-containing fractions were purified by HPLC (9: 68%B-64%B in 80 min, 3.4 mL min ^-1 ; 11: 67%B-62%B in 80 min, 3.4 mL min ^-1 ) providing the product as a white powder (9: 47 µg, 17%, 11: 50 µg, 10%). Analytical data NMR data was obtained at room temperature on a 600 MHz instrument from Bruker. The chemical shift δ is given in parts per million (ppm) and refers to tetramethylsilane and the residual solvent peak [ ¹ H-NMR: δ(D ₂ O) = 4.79 ppm]. NMR data is given as follows: ¹ H-NMR: chemical shift (multiplicity, coupling constants, relative integral, functional group); ¹³ C data are extracted from HSQC spectra and given as follows: chemical shift. Multiplicity is defined as follows: s = singlet; d = doublet; t = triplet; m = multiplet. Signals were assigned by numbering the monosacharide units starting at the reducing end of the oligosaccharide. Monosacharides attached to the mannose-6 branch are indicated by a “ ’ “ (prime) and those attached to the mannose-3 branch without any mark. The assignment was done by using corresponding 2D-NMR spectra (COSY, HSQC). Due to the use of ammonium formate containing buffers during final purification, several spectra show residual formic acid (8.46 ppm, not shown in the NMR spectra) contamination. The yield/concentration of the final products was determined by NMR spectroscopy, using n-propanol as an internal standard. High resolution masses were measured on an Agilent 6560 Ion Mobility Q-TOF LC-MS system. 2 ¹ H NMR (600 MHz, D ₂ O): δ = 5.13 (s, 1H, H-1, Man-4), 5.08 (d, J = 9.8 Hz, 1H, H-1, GlcNAc-1), 4.95 (s, 1H, H-1, Man-4’), 4.79 (s, 1H, H-1, Man-3), 4.64-4.59 (m, 3H, H- 1, GlcNAc-5, H-1, GlcNAc-5’), 4.56 (d, J = 8.4 Hz, 1H, H-1, GlcNAc-2), 4.46 (d, 1H, H-1, Gal-6’), 4.26 (s, 1H, H-2, Man-3), 4.20 (s, 1H, H-2, Man-4), 4.12 (s, 1H, H-2, Man-4’), 4.03-3.40 (m, 68H, H-2-H-6, GlcNAc-1, H-2-H-6, GlcNAc-2, H-3-H-6, Man- 3, H-3-H-6, Man-4, H-3-H-6, Man-4’, H-2-H-6, GlcNAc-5, H-2-H-6, GlcNAc-5’, H-2- H-6, Gal-6’, H-4-H-9, Neu5Ac-7’, αCH-Asn), 2.97-2.84 (m, 2H, βCH ₂ -Asn), 2.68 (dd, J = 12.4 Hz, 4.7 Hz, 1H, H-3eq, Neu5Ac-7’), 2.14-2.10 (m, 19H, NHAc), 1.73 (t, J = 12.2 Hz, 1H, H-3ax, NeuAc-7’). ¹³ C NMR from HSQC (150 MHz, D ₂ O): δ = 103.5, 101.3, 100.5, 99.5, 78.1, 96.9, 99.6, 70.2, 76.4, 76.2, 63.3, 60.4, 50.9, 65.8, 61.1, 69.3, 71.7, 53.6, 60.0, 73.6, 51.8, 65.9, 80.2, 54.8, 60.3, 72.5, 80.7, 78.7, 68.2, 59.8, 61.8, 74.3, 76.2, 68.4, 63.3, 70.7, 67.3, 69.9, 75.8, 35.0, 40.1, 22.3, 40.1. ESI- HRMS: for C ₇₁ H ₁₁₇ N ₇ O ₅₁ : m/z [M-2H] ^-2 ; calcd: 940.8316; found: 940.8288. 3 (Array #9) ¹ H NMR (600 MHz, D ₂ O): δ = 5.13 (s, 1H, H-1, Man-4), 5.08 (d, J = 9.8 Hz, 1H, H-1, GlcNAc-1), 4.96 (s, 1H, H-1, Man-4’), 4.78 (s, 1H, H-1, Man-3), 4.64-4.57 (m, 3H, H- 1, GlcNAc-2, H-1, GlcNAc-5, H-1, GlcNAc-5’), 4.49-4.44 (m, 2H, H-1, Gal-6, H-1, Gal-6’), 4.26 (s, 1H, H-2, Man-3), 4.20 (s, 1H, H-2, Man-4), 4.12 (s, 1H, H-2, Man-4’), 4.04-3.47 (m, 39H, H-2-H-6, GlcNAc-1, H-2-H-6, GlcNAc-2, H-3-H-6, Man-3, H-3-H- 6, Man-4, H-3-H-6, Man-4’, H-2-H-6, GlcNAc-5, H-2-H-6, GlcNAc-5’, H-2-H-6, Gal- 6, H-2-H-6, Gal-6’, H-4-H-9, Neu5Ac-7’, αCH-Asn), 2.97-2.84 (m, 2H, βCH ₂ -Asn), 2.68 (dd, J = 12.4 Hz, 4.6 Hz, 1H, H-3eq, Neu5Ac-7’), 2.11-1.99 (m, 17H, NHAc), 1.73 (t, J = 12.2 Hz, 1H, H-3ax, NeuAc-7’).13C NMR from HSQC (150 MHz, D ₂ O): δ = 99.5, 78.0, 78.1, 96.9, 100.6, 99.2, 101.3, 99.3, 103.1, 103.3, 70.2, 76.4, 76.2, 63.3, 60.1, 65.9, 51.0, 68.5, 61.7, 69.4, 71.7, 62.6, 53.6, 60.0, 73.8, 51.9, 65.8, 80.4, 60.9, 54.7, 72.3, 78.6, 78.7, 68.2, 61.8, 74.5, 68.3, 63.3, 70.8, 67.3, 35.3, 35.3, 39.9, 30.1, 22.3, 40.0. ESI-HRMS: for C77H127N7O56: m/z [M-2H] ^-2 ; calcd: 1021.8580; found: 1021.8564. 4 (Array #12) ¹ H NMR (600 MHz, D ₂ O): δ = 5.12 (s, 1H, H-1, Man-4), 5.08 (d, J = 9.6 Hz, 1H, H-1, GlcNAc-1), 4.95 (s, 1H, H-1, Man-4’), 4.78 (s, 1H, H-1, Man-3), 4.71 (d, J = 8.4 Hz, 1H, 1-H, GlcNAc-7), 4.64-4.56 (m, 3H, H-1, GlcNAc-2, H-1, GlcNAc-5, H-1, GlcNAc- 5’), 4.50-4.44 (m, 3H, H-1, Gal-6, H-1, Gal-6’, H-1, Gal-8), 4.26 (s, 1H, H-2, Man-3), 4.19 (s, 1H, H-2, Man-4), 4.16 (s, 2H, H-3, Gal-8), 4.12 (s, 1H, H-2, Man-4’), 4.06- 3.46 (m, 43H, H-2-H-6, GlcNAc-1, H-2-H-6, GlcNAc-2, H-3-H-6, Man-3, H-3-H-6, Man-4, H-3-H-6, Man-4’, H-2-H-6, GlcNAc-5, H-2-H-6, GlcNAc-5’, H-2-H-6, Gal-6, H-2-H-6, Gal-6’, H-4-H-9, Neu5Ac-7’, H-2-H-6, GlcNAc-7, H-2-H-6, Gal-8, αCH- Asn), 2.98-2.82 (m, 2H, βCH ₂ -Asn), 2.68 (dd, J = 12.9 Hz, 4.3 Hz, 1H, H-3eq, Neu5Ac-7’), 2.12-1.98 (m, 17H, NHAc), 1.73 (t, J = 12.2 Hz, 1H, H-3ax, NeuAc-7’). ¹³ C NMR from HSQC (150 MHz, D ₂ O): δ = 99.5, 78.1, 96.9, 100.5, 102.7, 101.3, 99.4, 103.1, 70.1, 76.4, 68.3, 76.2, 63.3, 50.9, 60.1, 65.8, 68.6, 69.1, 71.8, 62.4, 53.6, 60.0, 73.7, 51.9, 65.8, 54.8, 60.9, 72.3, 78.3, 82.0, 75.0, 80.6, 62.1, 61.7, 74.5, 70.0, 68.4, 63.3, 70.8, 67.3, 35.0, 40.1, 22.2, 40.1. ESI-HRMS: for C ₉₁ H ₁₅₀ N ₈ O ₆₆ : m/z [M-2H] ^-2 ; calcd: 1204.4241; found: 1204.4221. 5 (Array #13) ¹ H NMR (600 MHz, D ₂ O): δ = 5.10 (s, 1H, H-1, Man-4), 5.06 (d, J = 9.8 Hz, 1H, H-1, GlcNAc-1), 4.94 (s, 1H, H-1, Man-4’), 4.76 (s, 1H, H-1, Man-3), 4.72 (d, J = 7.7 Hz, 1H, 1-H, GlcNAc-7), 4.62-4.54 (m, 3H, H-1, GlcNAc-2, H-1, GlcNAc-5, H-1, GlcNAc- 5’), 4.49-4.41 (m, 3H, H-1, Gal-6, H-1, Gal-6, H-1, Gal-8), 4.24 (s, 1H, H-2, Man-3), 4.18 (d, J = 3.7 Hz, 1H, H-2, Man-4), 4.15 (d, J = 3.2 Hz, 1H, H-3, Gal-8), 4.11 (d, J = 3.4 Hz, 1H, H-2, Man-4’), 4.03-3.44 (m, 44H, H-2-H-6, GlcNAc-1, H-2-H-6, GlcNAc-2, H-3-H-6, Man-3, H-3-H-6, Man-4, H-3-H-6, Man-4’, H-2-H-6, GlcNAc-5, H-2-H-6, GlcNAc-5’, H-2-H-6, Gal-6, H-2-H-6, Gal-6’, H-4-H-9, Neu5Ac-7‘, H-2-H-6, GlcNAc-7, H-2-H-6, Gal-8, H-4-H-9, Neu5Ac-9, αCH-Asn), 2.96-2.83 (m, 2H, βCH ₂ - Asn), 2.66 (dd, J = 12.6 Hz, 4.5 Hz, 2H, H-3eq, Neu5Ac-9, H-3eq, Neu5Ac-7’), 2.13- 1.92 (m, 20H, NHAc), 1.71 (t, J = 12.2 Hz, 2H, H-3ax, NeuAc-7‘, H-3ax, Neu5Ac-9). ¹³ C NMR from HSQC (150 MHz, D ₂ O): δ = 99.5, 78.1, 96.9, 100.5, 102.6, 101.4, 99.4, 103.3, 70.2, 68.3, 76.2, 63.3, 50.9, 60.1, 65.8, 68.6, 69.3, 71.7, 62.4, 62.6, 53.5, 60.1, 73.6, 51.9, 65.7, 54.7, 60.7, 82.0, 72.4, 80.6, 68.2, 59.7, 62.5, 62.2, 74.4, 68.4, 63.3, 70.7, 67.3, 35.1, 35.0, 40.1, 22.2, 40.1. ESI-HRMS: for C ₁₀₂ H ₁₆₇ N ₉ O ₇₄ : m/z [M-3H] ^-3 ; calcd: 899.9814; found: 899.9787. 6 (Array #16) ¹ H NMR (600 MHz, D ₂ O): δ = 5.13 (s, 1H, H-1, Man-4), 5.08 (d, J = 9.8 Hz, 1H, H-1, GlcNAc-1), 4.95 (s, 1H, H-1, Man-4’), 4.78 (s, 1H, H-1, Man-3), 4.71 (d, J = 8.5 Hz, 2H, 1-H, GlcNAc-7), 4.65-4.57 (m, 4H, H-1, GlcNAc-2, H-1, GlcNAc-5, H-1, GlcNAc- 5’, 1-H, GlcNAc-9), 4.50-4.44 (m, 4H, H-1, Gal-6, H-1, Gal-6’, H-1, Gal-8, H-1, Gal- 10), 4.26 (s, 1H, H-2, Man-3), 4.20 (s, 1H, H-2, Man-4), 4.16 (s, 2H, H-3, Gal-8, H-3, Gal-10), 4.13 (s, 1H, H-2, Man-4’), 4.03-3.48 (m, 53H, H-2-H-6, GlcNAc-1, H-2-H-6, GlcNAc-2, H-3-H-6, Man-3, H-3-H-6, Man-4, H-3-H-6, Man-4’, H-2-H-6, GlcNAc-5, H-2-H-6, GlcNAc-5’, H-2-H-6, Gal-6, H-2-H-6, Gal-6’, H-4-H-9, Neu5Ac-7’, H-2-H-6, GlcNAc-7, H-2-H-6, Gal-8, H-2-H-6, GlcNAc-9, H-2-H-6, Gal-10, αCH-Asn), 2.97- 2.84 (m, 2H, βCH ₂ -Asn), 2.68 (dd, J = 12.4 Hz, 4.7 Hz, 1H, H-3eq, Neu5Ac-7’), 2.12- 2.00 (m, 23H, NHAc), 1.73 (t, J = 12.2 Hz, 1H, H-3ax, Neu5Ac-7’). ¹³ C NMR from HSQC (150 MHz, D ₂ O): δ = 99.5, 78.0, 96.9, 100.5, 102.7, 101.3, 99.3, 102.9, 70.2, 76.4, 68.3, 76.2, 63.3, 50.9, 59.9, 68.5, 61.7, 69.4, 71.7, 62.2, 62.6, 53.5, 59.9, 73.7, 51.9, 55.1, 65.7, 80.4, 80.3, 60.9, 54.7, 72.2, 72.2, 78.2, 82.0, 75.0, 72.4, 80.7, 78.6, 68.2, 59.8, 61.8, 74.5, 78.4, 69.9, 68.4, 63.3, 70.8, 67.3, 35.0, 35.0, 40.0, 22.1, 40.1. ESI-HRMS: for C ₁₀₅ H ₁₇₃ N ₉ O ₇₆ : m/z [M-3H] ^-3 ; calcd: 924.6603; found: 924.6602. 7 (Array #17) ¹ H NMR (600 MHz, D ₂ O): δ = 5.11 (s, 1H, H-1, Man-4), 5.06 (d, J = 9.8 Hz, 1H, H-1, GlcNAc-1), 4.94 (s, 1H, H-1, Man-4’), 4.77 (s, 1H, H-1, Man-3), 4.74-4.66 (m, 2H, H- 1, GlcNAc-7, H-1, GlcNAc-9), 4.63-4.55 (m, 3H, H-1, GlcNAc-2, H-1, GlcNAc-5, H-1, GlcNAc-5’), 4.48-4.42 (m, 4H, H-1, Gal-6, H-1, Gal-6’, H-1, Gal-8, H-1, Gal-10), 4.24 (s, 1H, H-2, Man-3), 4.18 (s, 1H, H-2, Man-4), 4.15 (s, 2H, H-3, Gal-6, H-3, Gal-6’), 4.11 (s, 1H, H-2, Man-4’), 4.02-3.47 (m, 45H, H-2-H-6, GlcNAc-1, H-2-H-6, GlcNAc- 2, H-3-H-6, Man-3, H-3-H-6, Man-4, H-3-H-6, Man-4’, H-2-H-6, GlcNAc-5, H-2-H-6, GlcNAc-5’, H-2-H-6, Gal-6, H-2-H-6, Gal-6’, H-2-H-6, GlcNAc-7, H-2-H-6, Gal-8, H- 2-H-6, GlcNAc-9, H-2-H-6, Gal-10, H-4-H-9, Neu5Ac-11, H-4-H-9, Neu5Ac-7’, αCH- Asn), 2.96-2.82 (m, 2H, βCH ₂ -Asn), 2.70-2.63 (m, 2H, H-3eq, Neu5Ac-7’, H-3eq, Neu5Ac-11), 2.16-1.98 (m, 22H, NHAc), 1.71 (t, J = 12.1 Hz, 2H, H-3ax, Neu5Ac-7’, H-3ax, Neu5Ac-11). ¹³ C NMR from HSQC (150 MHz, D ₂ O): δ = 99.5, 78.0, 96.9, 100.5, 102.6, 101.3, 99.2, 103.3, 70.2, 76.4, 68.3, 76.2, 63.3, 50.9, 60.0, 65.8, 68.4, 61.8, 69.4, 71.7, 62.6, 53.6, 60.0, 73.7, 51.9, 55.0, 65.7, 80.3, 54.7, 60.8, 82.0, 78.3, 74.8, 72.3, 51.9, 80.6, 68.2, 62.5, 74.4, 70.0, 68.4, 63.3, 70.7, 67.3, 34.9, 34.9, 40.1, 22.2, 22.1, 40.1. ESI-HRMS: for C116H190N10O84: m/z [M-3H] ^-3 ; calcd: 1021.6921; found: 1021.6904. 8 ¹ H NMR (600 MHz, D ₂ O): δ = 5.11 (s, 1H, H-1, Man-4), 5.06 (d, J = 9.8 Hz, 1H, H-1, GlcNAc-1), 4.92 (s, 1H, H-1, Man-4’), 4.77 (s, 1H, H-1, Man-3), 4.69 (d, J = 8.3 Hz, 1H, 1-H, GlcNAc-7), 4.63-4.54 (m, 3H, H-1, GlcNAc-2, H-1, GlcNAc-5, H-1, GlcNAc- 5’), 4.49-4.50 (m, 3H, H-1, Gal-6, H-1, Gal-6’, H-1, Gal-8), 4.24 (s, 1H, H-2, Man-3), 4.18 (s, 1H, H-2, Man-4), 4.15 (s, 1H, H-3, Gal-8), 4.10 (s, 1H, H-2, Man-4’), 4.01- 3.42 (m, 58H, H-2-H-6, GlcNAc-1, H-2-H-6, GlcNAc-2, H-3-H-6, Man-3, H-3-H-6, Man-4, H-3-H-6, Man-4’, H-2-H-6, GlcNAc-5, H-2-H-6, GlcNAc-5’, H-2-H-6, Gal-6, H-2-H-6, Gal-6’, H-2-H-6, GlcNAc-7, H-2-H-6, Gal-8, αCH-Asn), 3.00-2.82 (m, 2H, βCH ₂ -Asn), 2.12-1.96 (m, 17H, NHAc). ¹³ C NMR from HSQC (150 MHz, D ₂ O): δ = 99.5, 78.1, 97.1, 100.4, 100.4, 102.8, 102.8, 101.3, 101.3, 99.5, 102.9, 70.2, 76.4, 68.3, 76.3, 60.0, 65.8, 68.6, 61.7, 61.7, 69.4, 53.6, 59.9, 65.7, 61.0, 54.9, 72.2, 78.5, 75.2, 82.1, 72.5, 59.8, 61.7, 69.9, 74.6, 71.0, 67.3, 35.0, 22.3. ESI-HRMS: for C ₈₀ H ₁₃₃ N ₇ O ₅₈ : m/z [M-2H] ^-2 ; calcd: 1058.8764; found: 1058.8761. 9 (Array #11) ¹ H NMR (600 MHz, D ₂ O): δ = 5.09 (s, 1H, H-1, Man-4), 5.05 (d, J = 9.9 Hz, 1H, H-1, GlcNAc-1), 4.93 (s, 1H, H-1, Man-4’), 4.75 (s, 1H, H-1, Man-3), 4.67 (d, J = 8.4 Hz, 1H, 1-H, GlcNAc-7), 4.61-4.54 (m, 3H, H-1, GlcNAc-2, H-1, GlcNAc-5, H-1, GlcNAc- 5’), 4.47-4.40 (m, 3H, H-1, Gal-6, H-1, Gal-6’, H-1, Gal-8), 4.23 (s, 1H, H-2, Man-3), 4.17 (s, 1H, H-2, Man-4), 4.14 (s, 1H, H-3, Gal-8), 4.10 (s, 1H, H-2, Man-4’), 3.99- 3.44 (m, 36H, H-2-H-6, GlcNAc-1, H-2-H-6, GlcNAc-2, H-3-H-6, Man-3, H-3-H-6, Man-4, H-3-H-6, Man-4’, H-2-H-6, GlcNAc-5, H-2-H-6, GlcNAc-5’, H-2-H-6, Gal-6, H-2-H-6, Gal-6’, H-2-H-6, GlcNAc-7, H-2-H-6, Gal-8, Neu5Ac-7’/9, αCH-Asn), 2.94- 2.81 (m, 2H, βCH ₂ -Asn), 2.65 (d, J = 12.7 Hz, 1H, H-3eq, Neu5Ac-7’/9), 2.08-1.96 (m, 23H, NHAc), 1.70 (t, J = 12.1 Hz, 1H, H-3ax, NeuAc-7’/9). ¹³ C NMR from HSQC (150 MHz, D ₂ O): δ = 99.5, 78.0, 96.9, 100.4, 102.7, 101.2, 98.4, 99.4, 102.8, 102.8, 103.5, 70.1, 76.3, 76.4, 68.3, 68.3, 76.2, 63.2, 50.8, 59.9, 68.4, 61.6, 69.4, 71.7, 71.7, 59.0, 62.6, 59.9, 73.7, 54.9, 80.4, 54.7, 60.9, 72.1, 78.1, 82.0, 69.6, 72.4, 78.0, 80.6, 59.7, 61.6, 70.0, 74.5, 63.3, 70.8, 67.3, 34.9, 40.0, 40.0, 22.3, 22.0, 22.0, 20.0. ESI- HRMS: for C ₉₁ H ₁₅₀ N ₈ O ₆₆ : m/z [M-2H] ^-2 ; calcd: 1204.4241; found: 1204.4272. 10 ¹ H NMR (600 MHz, D ₂ O): δ = 5.11 (s, 1H, H-1, Man-4), 5.06 (d, J = 9.7 Hz, 1H, H-1, GlcNAc-1), 4.92 (s, 1H, H-1, Man-4’), 4.76 (s, 1H, H-1, Man-3), 4.69 (2 x d, J = 8.4 Hz, 2H, 1-H, GlcNAc-7, 1-H, GlcNAc-9), 4.64-4.51 (m, 3H, H-1, GlcNAc-2, H-1, GlcNAc-5, H-1, GlcNAc-5’, 1-H), 4.47-4.40 (m, 4H, H-1, Gal-6, H-1, Gal-6’, H-1, Gal-8, H-1, Gal-10), 4.24 (s, 1H, H-2, Man-3), 4.18 (s, 1H, H-2, Man-4), 4.15 (s, 2H, H-3, Gal-8, H-3, Gal-10), 4.10 (s, 1H, H-2, Man-4’), 4.00-3.44 (m, 51H, H-2-H-6, GlcNAc-1, H-2-H-6, GlcNAc-2, H-3-H-6, Man-3, H-3-H-6, Man-4, H-3-H-6, Man-4’, H-2-H-6, GlcNAc-5, H-2-H-6, GlcNAc-5’, H-2-H-6, Gal-6, H-2-H-6, Gal-6’, H-2-H-6, GlcNAc-7, H-2-H-6, Gal-8, H-2-H-6, GlcNAc-9, H-2-H-6, Gal-10, αCH-Asn), 2.96- 2.82 (m, 2H, βCH ₂ -Asn), 2.11-1.97 (m, 17H, NHAc). ¹³ C NMR from HSQC (150 MHz, D ₂ O): δ = 99.5, 78.0, 97.0, 100.4, 102.7, 101.3, 99.4, 102.9, 102.9, 70.2, 76.4, 68.3, 76.3, 50.9, 59.9, 68.5, 61.6, 69.4, 53.6, 59.9, 55.1, 65.7, 60.9, 54.9, 72.2, 78.3, 75.1, 82.0, 72.4, 59.9, 61.6, 70.0, 74.6, 70.9, 67.3, 35.0, 35.0, 22.2, 22.2. ESI- HRMS: for C ₉₄ H ₁₅₆ N ₈ O ₆₈ : m/z [M-2H] ^-2 ; calcd: 1241.4425; found: 1241.4440. 11 (Array #15) ¹ H NMR (600 MHz, D ₂ O): δ = 5.09 (s, 1H, H-1, Man-4), 5.05 (d, J = 9.6 Hz, 1H, H-1, GlcNAc-1), 4.92 (s, 1H, H-1, Man-4’), 4.76 (s, 1H, H-1, Man-3), 4.68 (d, J = 8.3 Hz, 2H, 1-H, GlcNAc-7), 4.61-4.54 (m, 4H, H-1, GlcNAc-2, H-1, GlcNAc-5, H-1, GlcNAc- 5’, 1-H, GlcNAc-9), 4.47-4.40 (m, 4H, H-1, Gal-6, H-1, Gal-6’, H-1, Gal-8, H-1, Gal- 10), 4.23 (s, 1H, H-2, Man-3), 4.17 (s, 1H, H-2, Man-4), 4.14 (s, 2H, H-3, Gal-8, H-3, Gal-10), 4.09 (s, 1H, H-2, Man-4’), 3.99-3.44 (m, 89H, H-2-H-6, GlcNAc-1, H-2-H-6, GlcNAc-2, H-3-H-6, Man-3, H-3-H-6, Man-4, H-3-H-6, Man-4’, H-2-H-6, GlcNAc-5, H-2-H-6, GlcNAc-5’, H-2-H-6, Gal-6, H-2-H-6, Gal-6’, H-4-H-9, Neu5Ac-7’/11, H-2- H-6, GlcNAc-7, H-2-H-6, Gal-8, H-2-H-6, GlcNAc-9, H-2-H-6, Gal-10, αCH-Asn), 2.95-2.81 (m, 2H, βCH ₂ -Asn), 2.65 (d, J = 12.3 Hz, 1H, H-3eq, Neu5Ac-7’/11), 2.11- 1.97 (m, 20H, NHAc), 1.70 (t, J = 12.3 Hz, 1H, H-3ax, NeuAc-7’/11). ¹³ C NMR from HSQC (150 MHz, D ₂ O): δ = 99.5, 78.0, 97.0, 100.4, 102.7, 101.2, 99.4, 102.9, 103.5, 76.3, 68.3, 76.8, 50.8, 59.9, 68.4, 71.1, 53.5, 59.9, 51.8, 55.0, 80.6, 60.9, 78.2, 72.1, 82.0, 78.5, 74.8, 72.4, 68.2, 80.6, 76.2, 74.5, 74.5, 69.9, 68.4, 70.8, 67.3, 35.0, 40.1, 22.1. ESI-HRMS: for C ₁₀₅ H ₁₇₃ N ₉ O ₇₆ : m/z [M-2H] ^-2 ; calcd: 1386.9902; found: 1387.0024. 12 (Array #8) ¹ H NMR (600 MHz, D ₂ O): δ = 5.12 (s, 1H, H-1, Man-4), 5.06 (d, J = 9.7 Hz, 1H, H-1, GlcNAc-1), 4.92 (s, 1H, H-1, Man-4’), 4.76 (s, 1H, H-1, Man-3), 4.63-4.54 (m, 3H, H- 1, GlcNAc-2, H-1, GlcNAc-5, H-1, GlcNAc-5’), 4.48-4.42 (m, 2H, H-1, Gal-6, H-1, Gal-6’), 4.25 (s, 1H, H-2, Man-3), 4.19 (s, 1H, H-2, Man-4), 4.10 (s, 1H, H-2, Man-4’), 4.02-3.44 (m, 47H, H-2-H-6, GlcNAc-1, H-2-H-6, GlcNAc-2, H-3-H-6, Man-3, H-3-H- 6, Man-4, H-3-H-6, Man-4’, H-2-H-6, GlcNAc-5, H-2-H-6, GlcNAc-5’, H-2-H-6, Gal- 6, H-2-H-6, Gal-6’, H-4-H-9, Neu5Ac-7, αCH-Asn), 2.96-2.81 (m, 2H, βCH ₂ -Asn), 2.66 (dd, J = 12.4 Hz, 4.7 Hz, 1H, H-3eq, Neu5Ac-7), 2.11-1.98 (m, 18H, NHAc), 1.71 (t, J = 12.2 Hz, 1H, H-3ax, NeuAc-7). ¹³ C NMR from HSQC (150 MHz, D ₂ O): δ = 99.5, 78.1, 97.0, 100.3, 101.3, 99.4, 103.1, 70.1, 76.3, 76.3, 60.1, 68.5, 61.6, 71.7, 53.6, 60.0, 73.6, 52.0, 54.7, 60.9, 72.4, 75.2, 78.9, 75.3, 72.4, 68.1, 80.7, 61.8, 74.4, 68.3, 63.4, 70.8, 67.3, 35.0, 40.1, 22.1. ESI-HRMS: for C ₇₇ H ₁₂₇ N ₇ O ₅₆ : m/z [M-2H] ^-2 ; calcd: 1021.8580; found: 1021.8575.

Virus production Materials: Eagle’s minimal essential medium (EMEM), penicillin, streptomycin, L-glutamine, sodium bicarbonate, HEPES, 1x non-essential amino acids and N-tosyl-L- phenylalanine chloromethyl ketone (TPCK) treated trypsin were purchased at Lonza Benelux BV, Breda, the Netherlands. Fetal bovine serum was obtained from Greiner. Madin-Darby canine kidney (MDCK) cells, MDCK-Siat cells and hCK cells were cultured in Eagle’s minimal essential medium supplemented with 10% fetal bovine serum (FBS), 100 U mL ^-1 penicillin (P), 100 U mL ^-1 streptomycin (S), 2 mM L- glutamine (L-glu), 1.5 mg mL ^-1 sodium bicarbonate (NaHCO3), 10mM HEPES, and 1x non-essential amino acids (NEAA) (5). In addition, hCK cells were supplemented with 2 µg mL ^-1 puromycin and 10 µg mL ^-1 blasticidin and MDCK- Siat cells were supplemented with 1 mg mL ^-1 Geneticine. To produce virus stocks, cells were washed twice with PBS one hour after inoculation with the virus of interest and cultured in infection media, consisting of EMEM supplemented with 100 U mL ^-1 penicillin, 100 μg mL ^-1 streptomycin, 2 mM glutamine, 1.5 mg mL- ¹ sodium bicarbonate, 10 mM Hepes, 1x non-essential amino acids, and 20 μg mL- ¹ N-tosyl-L-phenylalanine chloromethyl ketone (TPCK) treated trypsin. To produce virus stocks in eggs, 100 μL of virus was inoculated in the allantoic cavities of 11- day-old embryonated hens’ eggs. The allantoic fluid was harvested after 2 days. Microarray studies This write-up complies with MIRAGE Glycan Array Guideline v 1.0. Materials Virus isolates were produced as described above. Oseltamivir was purchased from Sigma Aldrich [Cat# SML1606]. CR8020 A/H3N2 stem antibody was kindly provided by Dr. Dirk Eggink and expressed following previously published procedures (6). Goat anti-human Alexa-647 [Cat# A21445] and streptavidin- AlexaFluor 635 [Cat# SA1011] antibodies were obtained from Thermo Fisher. Control lectins Erythrina cristagalli agglutinin (ECA) [Cat#B-1145], Sambuca nigra agglutinin (SNA) [Cat# B-1305], Maackia Amurensis Lectin I (Mal-I) [Cat# B-1315] were purchased from Vector Labs. Arrayer and printing surfaces Compounds were printed on amine reactive, NHS activated glass slides (NEXTERION ® Slide H) from Schott Inc using a Scienion sciFLEXARRAYER S3 non-contact microarray printer equipped with a Scienion PDC80 nozzle (Scienion Inc). Glycans were dissolved in printing buffer (sodium phosphate, 250 mM, pH 8.5) at a concentration of 100 µM. Each compound was printed in replicates of 6 with a spot volume of ~400 pL, at 20°C and 50% humidity. Slides were blocked with 5 mM ethanolamine in Tris buffer (pH 9, 50 mM) for 1 h at 50 °C and rinsed with DI water after printing. Glycan Microarray Quality control was performed using the plant lectins Erythrina cristagalli agglutinin (ECA, specific for terminal Gal), Sambuca nigra agglutinin (SNA, specific for 2,6-linked Neu5Ac) and Maackia Amurensis Lectin I (MAL-I, specific for 2,3-linked Neu5Ac) and is shown in Figure 2. Quality control of the CR8020 A/H3N2 Influenza hemagglutinin stem specific antibody specificity was performed by incubation of the antibody to the array as described below, in the absence of a virus (Figure 2). The printed library of compounds comprised the glycans described above (#7, #8, #10, #11, #12, #14, #15, #16) and published previously (#1-#6, #8, #13, #1) (7). Sample Virus isolates (25 µL) were diluted with PBS-T (PBS + 0.1% Tween, 25 µL) and applied to the array surface in the presence of oseltamivir (200 nM) in a humidified chamber for 1 h. It was followed by a succesive rinsing with PBS-T (PBS + 0.1% Tween), PBS and deionized water (2x) and dried by centrifugation. The virus- bound slide was incubated for 1 h with the CR8020 A/H3N2 Influenza hemaglutinin stem specific antibody (100 µL, 5 µg/ml in PBS-T) and washed according to previous washing procedure. A secondary goat anti-human Alexa-647 antibody (100 µL, 2 µg mL ^-1 in PBS-T) was applied, incubated for 60 min in a humidified chamber and followed by the washing steps as described above. The control lectins containing a biotin tag were visualized with Streptavidin- AlexaFluor635. Slides were dried by centrifugation after the washing step and scanned immediately. Detector and data processing The stained slides were scanned using an Innopsys Innoscan 710 microarray scanner at the appropriate excitation wavelength. To ensure that all signals were in the linear range of the scanner’s detector and to avoid any saturation of the signals various gains and PMT values were employed. Images were analyzed with Mapix software (version 8.1.0 Innopsys) and processed with our home written Excel macro. The average fluorescence intensity and SD was measured for each compound after exclusion of the highest and lowest intensities from the spot replicates (n=4). Glycomic analysis Materials Acetic acid [Cat# 5330010050], MS grade formic acid [Cat# 5330020050], 2- aminoanthrallic acid (2-AA) [Cat#10680], dimethyl sulfoxide (DMSO) [Cat# W387520] and sodium cyanoborohydride [Cat# 156159] were obtained from Sigma Aldrich. Trifluoro acetic acid (TFA) was purchased from Acros Organics [Cat#434161000]. MS-grade acetonitrile (MeCN) was obtained from Biosolve [Cat# 200-835-2]. PNGase F was purchased from Roche Diagnostics (1U defined as the amount of enzyme catalysing the conversion of 1 µmol(substrate)/min) [Cat#06538355103]. Denaturation buffer (0.5% SDS, 40 mM dithiothreitol (DTT)), 1% NP-40, and glycobuffer (50 mM sodium phosphate) were obtained from New England BioLabs. Ammonium formate was purchased from Fluka chemicals [Cat# AGG1946-85021], C18 solid phase extraction (SPE) Sep-Pak® Vac (1cc) columns from Waters Corporation [Cat# WAT054955], PGC SPE Hypercarb Hypersep (1cc) columns from Thermo Scientific [Cat# 60106-303] and PD Minitrap Sephadex G-10 size exclusion cartridges from GE Healthcare [Cat# 28-9180-10]. MilliQ water was obtained from a Synergy® water purification system. N-glycan extraction and release from erythrocytes Cell surface N-glycans were extracted according to a reported protocol (8). Briefly, erythrocytes (400 µL, 50%) were concentrated by centrifugation (430 rcf, 10 min) and removal of the supernatant. Erythrocytes were lysed under gentle shaking at room temperature for 60 minutes using deionized water (3x pellet size). The suspension was centrifuged (4000 rcf, 10 min), the supernatant removed, and the pellet resuspended in deionized water. The process was repeated until the pellet decolorized, indicating an efficient lysis of the erythrocytes. Denaturing of the cell membrane pellet was performed by heating for 10 min to 95 °C in denaturation buffer (0.5% SDS, 40mM DTT in H ₂ O). The N-glycans were released during overnight incubation at 37°C with PNGase F (5 U) in a sodium phosphate buffer (50 mM, pH 7.5) or with Endo F2 in sodium acetate buffer (50 mM, pH 4), both containing NP-40 (1%). Purification and labeling of N-glycans The released N-glycans were applied to a C18 SPE cartridge and glycans were eluted with 5% MeCN in H ₂ O (0.05% TFA, 1 mL). The eluate was further purified on a PGC SPE cartridge by gradually increasing the hydrophobicity from 100% H ₂ O (0.05% TFA, 1 mL) to 5% MeCN in H ₂ O (0.05% TFA, 1 mL), and to 50% MeCN in H ₂ O (0.05% TFA, 1 mL) which eluted the glycans. After drying in a N2 flow, the sample was dissolved in H ₂ O (10 μL) and labelled by addition of a solution (10 μL) of 2-AA (48 mg/mL) and sodium cyanoborohydride (63 mg/mL) in DMSO/acetic acid (10:3 v/v) for 2 h at 65 °C. The crude mixture was diluted with H ₂ O (80 uL) and purified on a Minitrap Sephadex G-10 gravity column by washing with of H ₂ O (700 µL) and eluting with H ₂ O (600 µL). The eluate was dried under N2 flow and dissolved in 20 µL of 50% MeCN in H ₂ O prior to LCMS analysis. HILIC-IMS-QTOF analysis of N-glycans and data treatment The N-glycan analysis was performed on a 1260 Infinity liquid chromatography system (Agilent Technologies) coupled to a 6560 IM-QTOF mass spectrometer (Agilent Technologies) and HPLC separation with a ZIC®-cHILIC column (3 µm, 100 Å, 150 mm x 2.1 mm, Merck) and a similar guard column (20 mm x 2.1 mm, Merck) using a gradient from 30% A to 50% A within 25 min (A: 50 mM (NH ₄ ) ₂ CO ₃ in H ₂ O; B: MeCN; 0.2 mL/min; 60 °C). MS analysis was performed with a drying gas temperature of 300 °C and a flow of 8 mL/min. The nebulizer pressure was set to 40 psi, the sheath gas flow to 11 L/min and the temperature to 350 °C. Measurements were run in negative mode with the capillary established at 3500 V. During the runs the Agilent tuning mix was infused for mass calibration based on reference signals at m/z 112.9855 and m/z 1033.9881. The data analysis was performed with the Mass Hunter IM-MS Browser and the find feature function filtering for masses with an ion intensity of ≥500. Found masses were processed with the online Glycomod tool to identify glycan related masses (9). Glyco-engineering of erythrocytes Materials Arthrobacter ureafaciens neuraminidase was purchased at New England Biolabs (1 U defined as the amount of enzyme catalyzing the conversion of 1 µmol(substrate)/min) [Cat# P0722L]. Mammalian glycosyltransferases were expressed according to literature reports (1). B3GNT2 and ST6Gal1 were cleaved and purified from GFP tags prior to use. Alkaline phosphatase (FastAP) was purchased at Thermo Scientific [Cat# EF0651]. Nucleotide sugars UDP-Gal, UDP- GlcNAc and CMP-Neu5Ac were obtained from Roche Diagnostics [UDP-Gal: Cat# 07703562103; UDP-GlcNAc: Cat# 06369855103; CMP-NeuAc: Cat# 05974003103]. Erythrocyte preparation Fresh blood from chicken or turkey was centrifuged (10 min, 430 rcf) followed by removal of the supernatant. Pellets were washed three times in PBS with intermittent centrifugation (430 rcf, 10 min). Erythrocyte solutions were stored in a 50% solution in PBS until further use. Enzymatic extension To a suspension of fowl erythrocytes (250 µL, 50%), PBS (900 µL) and Arthrobacter ureafaciens neuraminidase (12 u) were added. The cells were incubated for 6 h at 37 ºC while tilting. Next, glycosyltransferases B4GALT1 (37,5 µL, 1 mg/ml) and B3GNT2 (37,5 µL, 1 mg/ml), the nucleotide sugars UDP-Gal (4.4 mM) and UDP- GlcNAc (4.4 mM), alkaline phosphatase (6 units), MnCl ₂ (2 mM) and BSA (6 µL, 2 mg mL ^-1 ) were added. This reaction mixture was incubated overnight at 37 ºC while tilting. The erythrocytes were washed in PBS (2x, 600 µL) and the pellet was reconstituted in 900 µL PBS. Resialylation of the erythrocytes was performed using ST6Gal1 (37,5 µL, 1 mg mL ^-1 ) and CMP-Neu5Ac (4.4 mM) in the presence of alkaline phosphatase (6 u) and BSA (6 µL, 2 mg mL ^-1 ) for 4 h at 37 ºC while tilting. The erythrocytes were washed in PBS (1x, 600 µL) and diluted to a 1% solution for Hemagglutination (inhibition) assays. Stability assay Fresh fowl erythrocytes were glyco-engineered as described above or used unmodified. Hemagglutination assay was performed every two days using two viruses, A/NL/761/09 and A/NL/816/91. Hemagglutination assays were performed as described above in full biological triplicates. The means were plotted ± SEM and are shown in Figure 3. Hemagglutination assay Hemagglutination assays were performed following standard procedures (10). Briefly, virus stocks were two-fold serial diluted in the presence of oseltamivir (20 nM). Turkey erythrocytes (1%, 25 µL) were mixed with the serial diluted viruses and incubated for 1 h at 4 ºC before recording of the results. Titers were expressed as the highest dilution of virus stock that completely agglutinated the turkey erythrocytes. Hemagglutination inhibition assay Hemagglutination inhibition assays were performed following standard protocols (10). Briefly, for the preparation of the antisera, ferrets were inoculated intranasally and blood was obtained 14 d later. Antisera were pre-treated with receptor destroying enzyme (RDE) by incubating overnight with an in-house produced filtrate of Vibrio Cholera at 37 ºC followed by a 1 h incubation at 56 ºC. The treated antisera were pre-absorbed with extended turkey erythrocytes (10%) in two cycles of a 1 h incubation at 4 ºC. Pre-absorbed antisera were two-fold serial diluted (starting at 1:10) and mixed with virus stock (25 µL) containing 4 hemagglutinating units. Viruses were incubated with the antisera for 1 h at 4 ºC in the presence of BSA (0.25%) and oseltamivir (20 nM). Turkey erythrocyte solution (25 µL, 1%) was added and after 1 h incubation at 4 ºC inhibition patterns were recorded. Titers were expressed as the value of the highest serum dilution that gave complete inhibition of agglutination. Focus reduction assay Focus reduction assays were performed following standard protocols (11). First, infectious titers of the virus stocks were determined in hCK cells as described previously (12). RDE-treated sera were two-fold diluted in a 96-well plate (starting at 1:10) and mixed 1:1 with 100 TCID50/50 µL of virus. After 1 h incubation at 35 ºC, 100 µL of the mixtures were transferred to hCK cells and after 90 min incubation at 35 ºC, cells were washed and overlayed with 1.6% carboxymethylcellulose. After 48 h at 35 ºC, cells were washed and fixed with formalin and permeabilized using 0.5% Triton X-100 for 10 min at room temperature. Subsequently, immunostaining was performed using a mouse monoclonal antibody (HB65; EVL, Woerden, The Netherlands) directed against the viral nucleoprotein (NP), followed by a horseradish peroxidase-labeled goat anti mouse immunoglobulin preparation (GAM-HRPO, Invitrogen, Foster city, CA), both for 1 h at room temperature. After washing, True-Blue substrate (KPL, Gaitherburg, Maryland) was added followed by a 10 min incubation at room temperature. The plates were washed, dried, and submitted to automated image capture using a Series 6 ImmunoSpot Image Analyzer (CTL Immuno-Spot, Cleveland OH, USA) to quantitate the percentage well area covered by spots of infected cells. Inhibition ≥ 90% was considered positive for neutralization. References referred to in the above Materials & Methods section 1. K. W. Moremen et al., Expression system for structural and functional studies of human glycosylation enzymes. Nat Chem Biol 14, 156-162 (2018). 2. J. B. McArthur, H. Yu, J. Zeng, X. Chen, Converting Pasteurella multocida α2–3-sialyltransferase 1 (PmST1) to a regioselective α2–6-sialyltransferase by saturation mutagenesis and regioselective screening. Organic & biomolecular chemistry 15, 1700-1709 (2017). 3. A. Seko et al., Occurrence of a sialylglycopeptide and free sialylglycans in hen's egg yolk. Biochimica et Biophysica Acta (BBA) - General Subjects 1335, 23-32 (1997). 4. L. Liu et al., Streamlining the chemoenzymatic synthesis of complex N- glycans by a stop and go strategy. Nature Chemistry, (2018). 5. K. Takada et al., A humanized MDCK cell line for the efficient isolation and propagation of human influenza viruses. Nat Microbiol 4, 1268-1273 (2019). 6. D. C. Ekiert et al., A Highly Conserved Neutralizing Epitope on Group 2 Influenza A Viruses. Science 333, 843 (2011). 7. F. Broszeit et al., N-Glycolylneuraminic Acid as a Receptor for Influenza A Viruses. Cell Reports 27, 3284-3294.e3286 (2019). 8. U. Aich et al., Glycomics-based analysis of chicken red blood cells provides insight into the selectivity of the viral agglutination assay. FEBS J 278, 1699-1712 (2011). 9. C. A. Cooper, E. Gasteiger, N. H. Packer, GlycoMod–a software tool for determining glycosylation compositions from mass spectrometric data. PROTEOMICS: International Edition 1, 340-349 (2001). 10. WHO, Manual for the laboratory diagnosis and virological surveillance of influenza. (2011). 11. C. A. van Baalen et al., ViroSpot microneutralization assay for antigenic characterization of human influenza viruses. Vaccine 35, 46-52 (2017). 12. C. A. van Baalen et al., Detection of Nonhemagglutinating Influenza A(H3) Viruses by Enzyme-Linked Immunosorbent Assay in Quantitative Influenza Virus Culture. Journal of Clinical Microbiology 52, 1672-1677 (2014). Results We constructed a glycan array that is populated with bi-antennary N-glycans having different numbers of LacNAc repeating units. These compounds were either unmodified (compounds 1-3), capped by avian α2,3-linked (compounds 4-6) or human α2,6-linked sialosides (compounds 7-17). See Figure 4. To probe the importance of glycan architecture for HA recognition, we developed a chemo- enzymatic strategy that could provide 2,6-sialosides having such architectures (8, 9, 11, 12, 13, 15, 16 and 17). These compounds made it possible to probe the importance of mono- vs. bidentate binding interactions, preference for a specific antenna, and possible interference of a neighboring arm. The quality of the printing was validated by probing the array with the lectins ECA, SNA and MAL1 (Fig.2). The glycan array was probed with five A/H3N2 viruses representing different evolutionary time points and exhibiting a declining ability to agglutinate erythrocytes (Fig.4). The first three are A/NL/816/91 (NL91), which can agglutinate chicken and turkey erythrocytes, A/NL/109/03 (NL03) which only agglutinates turkey erythrocytes (Nobusawa et al.; Medeiros et al), A/NL/761/09 (NL09) that only agglutinates 2,6-resialylated turkey erythrocytes (Lin et al., 2012; Lin et al., 2010; Mögling et al.). We also used A/NL/1797/17 (NL17) and A/NL/371/19 (NL19) as very recent examples of viruses hemagglutinating all commonly used erythrocytes insufficiently for an HI assay as well as decreased efficiency to infect MDCK cells (Takada et al.). Whole viruses were applied to the microarray and detection of binding was accomplished by using a human anti-H3 stalk antibody (CR8020) (Fig. 4). NL91 recognized most of the human-type receptors, including compounds that have an α2,6-sialoside on a mono-LacNAc residue (Fig. 7A). Compound 8 exhibited a substantial greater responsiveness compared to 9 indicating this virus has a preference for a sialoside at the α1,3-arm. Interestingly, the sialyltransferase ST6Gal1, which is solely responsible for installing human-type receptors, preferentially modifies the α1,3-arm of N-linked glycans indicating the virus has evolved to recognize such structures. Compounds 7 and 8 did bind equally well demonstrating that an additional sialic acid at the α1,6-arm does not contribute to binding. Another unanticipated observation was that compounds 12 and 16 did not exhibit binding whereas 9 showed responsiveness highlighting that an extended and unmodified LAcNAc moiety at the α1,3-arm can block recognition of the other arm. Collectively, the results show an N-glycan having two LacNAc moieties modified by a single sialoside is the minimal epitope for NL91. NL03 and NL09 recognized far fewer glycans and did not bind to structures having their α2,6-sialosides at a mono-LacNAc moiety (7-11 and 15). This observation shows that the minimal receptor for this virus is a bi- antennary sialo-glycan that has at least on one arm an α2,6-sialylated di-LacNAc moiety. NL17 and NL19, which have evolutionary further progressed, showed strong responsiveness only to 14, 15 and 17. These glycans have in common that at least one of the arms is extended by three consecutive LacNAc units that is further modified by a α2,6-sialoside. Thus, a bi-antennary glycan, wherein at least one arm of said bi-antennary glycan comprises at least three LacNAc moieties, represents the minimal receptor for these viruses. Mono-sialylated derivative 15 gave a similar responsiveness on the array compared to bis-sialosides 14 and 17 indicating that a bidentate binding event does not substantially contribute to recognition as previously suggested (Peng et al.). Instead, it appears that reduction in binding of sialyl N-acetyl lactosamine has been compensated by recognizing sialosides at extended LacNAc residues. Example 2 Next, we examined structures of N-linked glycans expressed by chicken and turkey erythrocytes and compared the data with the receptor requirements of the various A/H3N2 viruses. Membrane fractions of the cells were treated with PNGase F to release the N-glycans which were isolated by solid phase extraction using C18 and Porous Graphitized Carbon (PGC) cartridges and then analyzed by liquid chromatography mass spectrometry (LC-MS) (Aich et al.). The 30 most abundant complex type N-glycan compositions for the two cell types are presented in Fig. 7B. Strikingly, chicken erythrocytes did not substantially express N-glycans with at least four LacNAc units. Turkey erythrocytes did express some glycans with this number of LacNAc repeating units, but the majority was assigned as tri- and tetra- antennary glycans because of the presence of three or four sialic acids. The latter was supported by selective releasing bi-antennary complex type N-glycans from membrane fractions using Endo F2, which only cleaves bi-antennary glycans, and in this case LC-MS analysis did not detect glycans having four LacNAc moieties (Fig.6). Thus, turkey erythrocytes also do not substantially display sialylated epitopes having three consecutive LacNAc moieties. Chicken erythrocytes expressed substantial quantities of high mannose glycans (Fig.8) whereas turkey cells displayed almost none of these structures. The greater abundance of complex type glycans on turkey erythrocytes offers a possible rationale for the ability of the NL03 and NL09 A/H3N2 viruses to agglutinate respectively unmodified or α2,6- resialylated turkey erythrocytes. Example 3 We embarked on a strategy to enzymatically remodel glycans of fowl erythrocytes to install receptors for contemporary A/H3N2 viruses to make them suitable for HI assays (Fig.5A). Treatment of erythrocytes with the neuraminidase of arthrobacter ureafaciens removes sialic acids and reveals terminal galactosides which are appropriate acceptors for installing additional LacNAc moieties. The latter residues can be introduced by the concerted action of the enzymes B4GalT1 and B3GnT2, which sequentially install β1,4-linked galactoside and β1,3-linked N- acetyl-glucosamine, respectively. The terminal galactosides of the resulting extended LacNAc moieties can then be modified by the sialyltransferase ST6Gal-I to install terminal α2,6-linked sialosides (Higa et al.). The enzymatic remodeling was conveniently performed by incubating the erythrocytes with the neuraminidase for 6 h after which recombinant B4GalT1 and B3GnT2, UDP-Gal and UDP-GlcNAc were added followed by incubation overnight (Moremen et al.). Next, the cells were pelleted by centrifugation, washed to remove the enzymes and sugar nucleotides and then incubated with ST6Gal1 in the presence of CMP- Neu5Ac for 4 h. Glycomic analysis of the resulting cells, which were denoted as 2,6- Sia Poly-LN cells, confirmed that the antennae of the N-linked glycans had been extended by additional LacNAc moieties, and both cell types expressed sialylated structures having four LacNAc units (Fig.5B). Analysis of glycans on turkey erythrocytes released by endo-F2 treatment confirmed the presence of bi-antennary glycans that have four LacNAc moieties and are potentially suitable receptors for contemporary H3N2 viruses (Fig. 6). Chicken and turkey erythrocytes express a mixture of α2,3- and α2,6-linked sialosides. To examine whether 2,6-resialylation alone would improve agglutination, control cells were prepared by treatment of arthrobacter ureafaciens neuraminidase and resialylated with ST6Gal1, (denoted as 2,6-Sia cells). As an additional control, we employed cells that have extended LacNAc moieties (denoted as Poly-LN cells) but lack sialic acids. Phenotypic properties of the glyco-engineered erythrocytes were examined using the hemagglutination (HA) assay (Fig. 5C). As expected, NL91 agglutinated unmodified, the 2,6-Sia and the 2,6-Sia-poly-LN erythrocytes, which was in agreement with the finding that these viruses can employ N-glycans that have simple and extended 2,6-sialylated structures. NL03 agglutinated unmodified turkey erythrocytes, but interestingly also 2,6-resialylated chicken erythrocytes. The latter may be due to an increase in abundance of 2,6-linked sialosides having two consecutive LacNAc repeats, which are present on chicken erythrocytes. 2,6- Resialylation of turkey erythrocytes was sufficient to recover agglutination of NL09, and in this case the greater abundance of 2,6-sialylation on extended structures already present on these cells may have improved agglutination. Importantly, NL17 and NL19 agglutinated only erythrocytes that were enzymatically remodeled to have extended sialylated LacNAc moieties (2,6-Sia Poly-LN cells). Although treatment of the erythrocytes with B4GalT1 and B3GNT2 resulted in only a relatively small fraction of bi-antennary glycans having four LacNAc moieties, this was sufficient for these cells to be agglutinated by the recent A/H3N2 viruses. Example 4 Next, HA assays were performed with a collection of A/H3N2 viruses (Table 2) to validate the robustness of the glyco-engineering method, and the focus was on contemporary H3N2 viruses that have lost the ability to hemagglutinate unmodified erythrocytes and do not replicate efficiently in wild-type MDCK cells. Several recent vaccine strains heavily adapted to growing in eggs (X-161B, IVR-147, X-223A, NIB-104, NIB-112 and X-327) and an A/H1N1 virus (IVR-180) and an influenza B strain (BX-69A) were included as controls. Except for the vaccine strains, almost all examined isolates exhibited substantially higher HA titers when erythrocytes were employed having extended sialylated LacNAc moieties (2,6-Sia Poly-LN cells). As expected, pre-2000 A/H3N2 strains efficiently agglutinated unmodified chicken, turkey and guinea pig erythrocytes. Importantly, A/H3N2 viruses that emerged after 2010 only agglutinated the 2,6-Sia Poly-LN cells having extended sialylated epitopes. Although some contemporary A/H3N2 viruses can agglutinate turkey or guinea pig erythrocytes when applied undiluted, especially the two 20193C3a viruses, extended sialylated LacNAc moieties increased the efficiency of agglutination by 3 to 10-fold, including the H1N1 and B controls. We performed a time course to determine the stability of the glyco- engineered cells and for up to three weeks no loss in titer or autoagglutination was observed (Fig. 3). The 2,6-Sia Poly-LN cells were next employed for antigenic characterization of typical recent seasonal A/H3N2 viruses of various clades by HI assay using post- infection ferret sera (Table 1). All antisera showed robust inhibition of the homologous viruses and variable inhibition of heterologous viruses. Results obtained with the egg-derived vaccine strains displayed poor correspondence with data generated with cell-passaged viruses of the same clade. Antisera raised against cell-passaged virus isolates generally showed greater clade specificity compared to egg-derived vaccine strains and vaccine strains showed generally broader sensitivity to inhibition than observed with cell-passaged viruses. As a consequence, The HI assays revealed that antisera raised against recent vaccine viruses, including A/Kansas/14/17 that was selected for the 2019/2020 northern hemisphere influenza vaccine (WHO, 2019), exhibited only minimal cross reactivity against circulating viruses from the same clades, indicating that the circulating viruses differ antigenically from the vaccine strains of the same clade. The results of the HI assay were compared with a focus reduction assay (FRA) using the same sera and viruses (Table 3). The FRA assay was introduced as an alternative for the HI assay when strains started losing hemagglutination potential (Jorquera et al.). In the FRA assay, the ability of antibodies to block virus infection in mammalian cell culture (MDCK-Siat cells) is quantified. The FRA confirmed the trends observed in the HI assay (Fig.9), indicating that the modified erythrocytes were reliable for the use for antigenic characterization of A/H3N2 viruses. Since their introduction in humans, A/H3N2 viruses have rapidly evolved to escape host immune pressure and as a result their receptor usage has changed, resulting in poor binding to N-glycans carrying 2,6-sialyl moieties on simple LacNAc structures. Our results show that these viruses have compensated for this poor binding by progressively recognizing 2,6-sialosides with longer LacNAc moieties. Glycomic analysis demonstrated that these extended receptors are not present on chicken erythrocytes and in very low abundance on turkey erythrocytes, providing a rationale for the lack of agglutination. The resulting failure of the HI assay is greatly complicating monitoring of antigenic changes of circulating A/H3N2 viruses. In combination with a lack of availability of suitable vaccine-production substrates and cocirculation of antigenically diverse A/H3N2 virus variants, the production of influenza vaccines with effectiveness against A/H3N2 viruses has been shown to be a challenge in the art. By employing an exo-enzymatic cell surface glycan engineering strategy, we were able to introduce appropriate glycan receptors on the cell surface of fowl erythrocytes allowing easy antigenic characterization of recent H3N2 isolates using the HI assay. Results from this assay confirmed that antigenically distinct viruses are circulating in humans and that egg-passaged A/H3N2 vaccine components match poorly to these circulating strains.

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ble 1. Hemagglutination inhibition assay.

e 2. Hemagglutination assay of A/H3N2 virus isolates using unmodified and Sia Poly-LacNAc fowl erythrocytes

Table 3. Focus reduction assay of recent A/H3N2 virus isolates.