The hemagglutinin (HA) protein is actually a combination of three identical proteins that are bound together to form an elongated cylindrical shape. HA protein consists of a globular head and a stem. The globular head consists of three chains, Chains A, C, and E. The stem of the protein consists of three chains as well, Chains B, D, and F. details of HA protein structure
Hemagglutinin (HA) protein is an antigenic glycoprotein found on the surface of the influenza viruses, responsible for binding the virus to the cell that is being infected. The name "hemagglutinin" comes from the protein's ability to cause red blood cells (erythrocytes) to clump together ("agglutinate") in vitro. details of HA protein function
The influenza A virus can be subdivided into different serotypes based on the type of hemagglutinin and neuraminidase. The "H" in "H1N1" for example refers to hemagglutinin (HA) and "N" in "H1N1" refers to neuraminidase (NA). There are 16 different types of HA and 9 different types of NA, therefore, there are potentially 144 different subtypes of influenza A viruses. Among them, two subtypes of influenza A, H1N1 and H3N2, most commonly infect humans. details of HA subtypes and flu strains
