Haemagglutinin viral surface protein. Molecular model of haemagglutinin, a surface protein from the influenza virus, complexed with a neutralising antibody. Haemagglutinin's function is to bind to the surface of its target cell and allow the viral genes into the cell, where replication occurs. There are 16 different types of haemagglutinin known, numbered 1-16. H1, H2 and H3 are found in human flu viruses, the rest mainly affect birds.