Hemagglutinin protein (influenza) (HA)

The **hemagglutinin protein** (HA) is a trimeric glycoprotein embedded in the envelope of influenza viruses, and is a major determinant of viral infectivity and immunogenicity[1][4][7][9]. Each monomer comprises two subunits, HA1 and HA2, which mediate distinct functions: HA1 binds to sialic acid-containing receptors on host cells, enabling virus attachment, while HA2 triggers membrane fusion upon acidification in the endosome, allowing viral RNA to enter the host cytoplasm[1][6][7][9]. HA is the primary target of neutralizing antibodies and underlies the antigenic classification of influenza A viruses into subtypes (e.g., H1, H3, H5)[4][7]. Its structural configuration and antigenic variability are central to influenza pathogenesis, pandemic potential, and annual vaccine development[1][4][6]. Drugs targeting HA’s function are under investigation, and antibody titers to HA serve as important biomarkers for vaccine efficacy and immune status[4].

Other names

HaemagglutininInfluenza hemagglutininInfluenza HAviral hemagglutinin

Blockade of receptor binding (antibodies binding the head domain prevent virus attachment) Inhibition of membrane fusion (antibodies or experimental fusion inhibitors destabilize the fusion-competent conformation) Immune neutralization (antibodies neutralize infectivity by blocking HA activity)