HA1 (hemaggulutinin1) belongs to the influenza viruses hemagglutinin family. Influenza hemagglutinin (HA) or haemagglutinin is a type of hemagglutinin found on the surface of the influenza viruses. It is an antigenic glycoprotein. It is responsible for binding the virus to the cell that is being infected. HA protein has two functions. Firstly, it allows the recognition of target vertebrate cells, accomplished through the binding of these cells' sialic acid-containing receptors.