Hemagglutinin (HA) is a surface protein found on influenza (flu) viruses. HA plays an important role in infection by allowing a flu virus to enter a healthy cell. HA is also an antigen. Antigens are molecular structures on the surface of viruses that are recognized by the immune system and are capable of triggering an immune response (i.e., the creation of antibodies). These antibodies protect the host from re-infection with the same influenza virus in the future. Therefore, HA is the active component (antigen) of inactivated flu vaccines. Most seasonal flu vaccines are designed to target the HA of the flu viruses that research suggests will be most common during the flu season.