Hemagglutinin protein of group 1 influenza A virus (HA)

Hemagglutinin is the most abundant glycoprotein on the surface of influenza A viruses and is essential for viral infectivity. It exists as a homotrimer, each monomer containing a globular "head" (receptor-binding domain, RBD) and a stalk (fusion domain). HA enables the virus to bind to sialic acid receptors on host cells, mediates virus–host membrane fusion upon endosomal acidification, and is cleaved into HA1 and HA2 subunits for activation. Group 1 hemagglutinins include several subtypes (most notably H1, H2, H5) and are especially important for pandemic surveillance and vaccine design due to their role in antibody recognition and escape. Hemagglutinin is the principal viral antigen for seasonal and pandemic influenza vaccine development, with ongoing research to design therapeutics and broadly neutralizing antibodies against conserved regions of this protein. If further subtyping information (e.g., H1, H5) is required, note that it applies to the "group 1" family, which includes canonical influenza A HA subtypes (H1, H2, H5, H6, H8, H9, H11, H12, H13, H16).

Other names

Hemagglutinin (influenza)Influenza A hemagglutininGroup 1 hemagglutinin (sometimes classified via H1, H2, H5, H6, H8, H9, H11, H12, H13, H16 subtypes)HA glycoproteinH1 hemagglutinin (for H1 strains specifically)

Inhibition of receptor binding—antibodies or designed ligands block sialic acid interaction; Inhibition of membrane fusion—small molecules or antibodies target the stem to prevent the conformational change needed for fusion; Neutralization of viral entry—antibody binding leads to virus neutralization, preventing infection.