Influenza A H1N1 hemagglutinin protein

The hemagglutinin (HA) protein is a trimeric glycoprotein found on the surface 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 homotrimer, with each monomer composed of two subunits: HA1 and HA2. The globular head domain (mainly HA1) contains the receptor-binding site and is highly antigenic. The stem or stalk domain (mainly HA2) anchors the protein in the viral membrane and mediates membrane fusion. Hemagglutinin is highly immunogenic; it stimulates neutralizing antibody responses that are central to vaccine efficacy. Antigenic drift (mutation) or shift (reassortment) in this protein can lead to immune escape or pandemics. Determines species specificity by recognizing different linkages of sialic acid residues present on avian versus mammalian cells. Plays a major role in virulence; mutations affecting its stability or activation pH can alter transmissibility and pathogenicity. Hemagglutination inhibition assays measure antibodies against this protein as a correlate of immunity/vaccine response. Recombinant forms are widely used for research, diagnostics, vaccine development, and structural studies.

Hemagglutinin is the target of neutralizing antibodies, preventing viral entry and infection.