Subtypes
There are at least 17 different HA antigens. These subtypes are named H1 through H17. H16 was discovered only in 2004 on influenza A viruses isolated from black-headed gulls from Sweden and Norway. The most recent H17 was discovered in 2012 in fruit bats. The first three hemagglutinins, H1, H2, and H3, are found in human influenza viruses.
Viral neuraminidase (NA) is another protein found on the surface of influenza. Influenza viruses are characterised by the type of HA and NA that they carry; hence H1N1, H5N2 etc.
A highly pathogenic avian flu virus of H5N1 type has been found to infect humans at a low rate. It has been reported that single amino acid changes in this avian virus strain's type H5 hemagglutinin have been found in human patients that "can significantly alter receptor specificity of avian H5N1 viruses, providing them with an ability to bind to receptors optimal for human influenza viruses". This finding seems to explain how an H5N1 virus that normally does not infect humans can mutate and become able to efficiently infect human cells. The hemagglutinin of the H5N1 virus has been associated with the high pathogenicity of this flu virus strain, apparently due to its ease of conversion to an active form by proteolysis.
Read more about this topic: Hemagglutinin (influenza)