Influenza A virus hemagglutinin HA1 chain
Hemagglutinin (HA) in influenza A has at least 18 different subtypes. These subtypes are named H1 through H18. The first three hemagglutinins, H1, H2, and H3, are found in human influenza viruses. By phylogenic similarity, the HA proteins are divided into 2 groups, with H1, H2, H5, H6, H8, H9, H11, H12, H13, H16, H17, and H18 belonging to group 1 and the rest in group 2.