Influenza hemagglutinin (HA) is a homotrimeric glycoprotein found on the surface of influenza viruses and is integral to its infectivity. Hemagglutinin is a Class I Fusion Protein, having multifunctional activity as both an attachment factor and membrane fusion protein. Therefore, HA is responsible for binding Influenza virus to sialic acid on the surface of target cells, such as cells in the upper respiratory tract or erythrocytes, following which event the virus is internalized. HA in influenza A has at least 18 different subtypes. These subtypes are named H1 through H18. 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. The serotype of influenza A virus is determined by the Hemagglutinin (HA) and Neuraminidase (NA) proteins present on its surface. Neuraminidase (NA) has 11 known subtypes, hence influenza virus is named as H1N1, H5N2 etc., depending on the combinations of HA and NA.