Influenza A virus hemagglutinin H3 subtype (HA (H3 subtype))

The **Influenza A virus hemagglutinin H3 subtype** is a trimeric viral envelope glycoprotein and the primary surface antigen of H3N2 influenza A viruses. It mediates two critical steps of the viral life cycle: (1) **receptor binding**, via its globular head domain, to sialic acid-containing receptors on host respiratory epithelial cells, and (2) **membrane fusion**, via its stem domain, allowing viral RNA to enter the host cell cytoplasm[1][3][7][9]. HA is synthesized as a precursor (HA0) and cleaved into HA1 and HA2 subunits, a process essential for fusion activation[3][7][9]. This protein is the major focus of immune recognition and vaccine design, but it is also highly variable due to antigenic drift and shift, which enables the virus to evade host immunity[6][8][10]. The evolution of the H3N2 HA receptor-binding site has resulted in altered receptor preferences and is a primary driver of seasonal vaccine updates and ongoing pandemic risk[5][8][10]. Key features include: - **Molecular trimeric structure** (each monomer with a head and stem domain). - **Major target of neutralizing antibodies and seasonal influenza vaccines**. - **Striking evolutionary adaptability** leading to frequent changes in antigenicity and receptor binding specificity.

Other names

Influenza hemagglutinin H3H3N2 hemagglutininInfluenza A H3N2 HAH3 hemagglutinin

Antibodies: Neutralization by blocking receptor binding and/or fusion process Vaccines: Elicit immune response against HA surface antigens to prevent infection Experimental small molecules: Inhibit conformational changes in HA needed for fusion (mostly preclinical)