Haemagglutinin viral surface protein. Molecular model of haemagglutinin, a surface protein from the influenza virus, complexed with a neutralising antibody. Haemagglutinin is found on the surface of many bacteria and viruses, notably the influenza virus. Its function is to bind to the surface of its target cell and allow the viral genes into the cell, where replication occurs. The protein is composed of three identical sub-units, which have a bulbous head borne on a long stalk. The colours of this molecule show the sub-units, and the head region of each sub-unit has been coloured differently to the stalk. 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, although the H5N1 strain of flu has killed dozens of humans.