Hemagglutinin of Influenza A Virus H5N1 Subtype (None)

Hemagglutinin (HA) is a surface glycoprotein and the principal antigenic determinant of the influenza A virus, including the H5N1 subtype. It plays a critical role in viral entry by mediating both attachment to host cell receptors and subsequent membrane fusion. The HA protein is also central to determining host specificity, pathogenicity, and immune response. HA binds to sialic acid-containing receptors on host cells and after endocytosis into host cells, low pH triggers conformational changes in HA that drive fusion between viral and endosomal membranes. Major antigenic sites are located on the globular head; these regions are targets for neutralizing antibodies but also undergo frequent mutation (antigenic drift). Highly Pathogenic Avian Influenza (HPAI) strains like A(H5N1) possess polybasic cleavage sites within their HA proteins. This allows ubiquitous cellular proteases to activate them efficiently contributing to systemic infection potential in birds and increased virulence in mammals. Mutations affecting its structure—especially at receptor binding or cleavage sites—can dramatically alter transmissibility between species.

Attachment to host cell receptors; mediation of membrane fusion