Influenza A virus hemagglutinin antigen (HA)

Influenza A virus hemagglutinin antigen is a homotrimeric glycoprotein found on the surface of influenza viruses. It is essential for viral infectivity by mediating two key steps in infection: binding to sialic acid-containing receptors on host cells (primarily respiratory epithelial cells) and facilitating fusion between the viral envelope and endosomal membranes after uptake into the cell. This process allows release of the viral genome into the cytoplasm. Hemagglutinin is highly immunogenic and serves as a primary target for neutralizing antibodies generated by natural infection or vaccination. There are at least 18 known subtypes (H1–H18), with different subtypes associated with human versus animal infections. Mutations in this protein can alter species specificity, pathogenicity, and resistance to immunity—making it central both as a therapeutic/vaccine target and as a driver of influenza epidemiology.

Other names

Hemagglutinin (influenza)Influenza hemagglutininHA glycoproteinH antigen (with subtype, e.g., H1, H3)

Neutralizing antibodies bind to the receptor-binding site or stem region of HA, blocking viral entry by preventing attachment or membrane fusion