Hemagglutinin Protein of Influenza A Virus H1N1 Subtype (HA)

Hemagglutinin (HA) is a major surface glycoprotein of the influenza A virus, including the H1N1 subtype. It is essential for viral infectivity, mediating both attachment to host cells and fusion of viral and cellular membranes. HA exists as a homotrimeric integral membrane glycoprotein, with each trimer composed of three identical monomers originating from a single precursor polypeptide (HA0), which is cleaved into HA1 and HA2 subunits. The globular head (mainly HA1) contains the receptor-binding site, while the stem (mainly HA2) contains the fusion peptide. HA is the primary target for neutralizing antibodies and a key antigen in influenza vaccines. Because it mediates critical steps in infection—attachment and entry—HA is targeted by neutralizing antibodies that block either receptor binding (head-directed) or conformational changes required for fusion (stem-directed). There are at least 16 known subtypes (H1–H16) based on sequence variation in hemagglutinin genes. H1N1 refers specifically to viruses with type 1 hemagglutinin paired with type 1 neuraminidase.

Other names

H1N1 HemagglutininHA proteinInfluenza HA

Neutralizing antibodies block receptor binding or conformational changes required for fusion; Fusion inhibitors target the HA2 subunit.