Haemagglutinin protein subunit. Molecular model of the ectodomain of the haemagglutinin HA(2) subunit. Haemagglutinin is a surface protein from the influenza A virus. 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.