Influenza A and B hemagglutinin glycoproteins (HA)

Influenza A and B hemagglutinin (HA) glycoproteins are the primary surface antigens of the influenza virus and the essential immunogenic components of the Afluria Quadrivalent vaccine (FDA, 2023). These proteins function as lectins that bind to specific sialic acid residues on host cell membranes, facilitating viral attachment and subsequent entry via receptor-mediated endocytosis (UniProt, 2024). Following endosomal acidification, the HA molecule undergoes a dramatic conformational change that mediates the fusion of the viral envelope with the host endosomal membrane, releasing the viral genome into the cytoplasm. In the context of vaccination, these glycoproteins serve as the primary targets for the host's immune system, eliciting the production of neutralizing antibodies that typically target the highly variable globular head region of the HA protein (CDC, 2023). Afluria Quadrivalent specifically contains purified HA from four distinct influenza strains—two Influenza A subtypes (H1N1 and H3N2) and two Influenza B lineages—to provide broad protection against circulating seasonal viruses. Because the HA protein frequently undergoes antigenic drift, the specific glycoproteins included in the vaccine are updated annually based on global surveillance and World Health Organization recommendations (WHO, 2024).

Other names

HemagglutininHAInfluenza virus surface glycoproteinViral hemagglutininHA1/HA2 subunit

The vaccine induces the production of strain-specific IgG antibodies that bind to the globular head of the hemagglutinin protein, sterically hindering the interaction between the virus and host cell sialic acid receptors, thereby neutralizing viral infectivity and preventing the initiation of infection (FDA Afluria Package Insert; CDC, 2023).