Haemagglutinin is an antigenic protein on the surface of the influenza virus, that enables it to bind to a receptor on the target cell. It is represented by the " H " found in the different forms of influenza (H5N1, H1N1, H2N2…). In reality there are 19 different forms of haemagglutinin numbered from 1 to 19. Its name comes from its ability to agglutinate red blood cells.
Simplified three-dimensional structure of haemaggluttinin. It is approximately 13.5 nm long. © National Institute of Health, Wikipedia, cc by sa 3.0