Roll of hemagglutinin gene in the biology of avian influenza virus

The hemagglutinin (HA), the major envelope glycoprotein of influenza, plays an important role during the early stage of infection, and changes in the HA gene prior to the emergence of pathogenic avian influenza viruses. The HA protein controls viral entry through membrane fusion of the viral envelope with the host cell membrane and allows the genetic information released to initiate new virus synthesis. Sharp antigenic variation of HA remains the critical challenge to the development of effective vaccines. Therefore, we highlight the role of HA in need of review: structure of HA, the fusion process and the HA receptor binding specificity in interspecies transmission and the impact of multiple mutations at antigenic sites and host antibodies to the parental virus, and the host susceptibility to productive infection by the drift strains.