Hemagglutinin protein of H1N1 influenza virus (HA (for general hemagglutinin); H1 HA is typical shorthand for this subtype)

The **hemagglutinin protein of H1N1 influenza virus** (H1 HA) is a trimeric glycoprotein embedded in the viral envelope of influenza A viruses, essential for viral entry into host cells[1][4][6]. Each monomer is synthesized as a precursor (HA0) and cleaved into two subunits, HA1 and HA2, which remain linked and form the mature functional protein[3][6]. HA mediates attachment by binding to sialic acid residues on host cell surfaces (via its globular head domain) and triggers envelope fusion with the endosomal membrane (via structural changes in the stem domain) after endocytosis and acidification[1][5]. It is the primary viral antigen targeted by neutralizing antibodies, vaccines, and experimental therapeutics, and is responsible for both seasonal and pandemic influenza A outbreaks (notably 1918 and 2009 for H1N1)[1][3][4][6]. The variability in its antigenic domains drives the need for continual vaccine updates and complicates antiviral strategies.

Other names

Influenza hemagglutininH1 hemagglutininH1 HAHA proteinInfluenza A virus hemagglutinin

Blockade of receptor binding (by antibodies or small molecules); Inhibition of membrane fusion step (by fusion inhibitors or neutralizing antibodies); Induction of immune response (vaccines elicit anti-HA antibodies)