Hemagglutinin protein of Influenza A (HA)

Hemagglutinin protein of Influenza A (HA) is a trimeric surface glycoprotein found on the influenza A virus envelope and is essential for mediating viral entry into host cells. Each HA monomer is synthesized as a single polypeptide (HA0) that is cleaved into HA1 and HA2 subunits, which remain covalently linked. The globular head (HA1) binds sialic acid-containing receptors on host cells, determining host range and tissue tropism, while the stalk domain (HA2) mediates membrane fusion following endocytosis and acidification. HA is also the primary antigenic determinant of the virus and a major target of the adaptive immune response, making it the focus of influenza vaccine development and therapeutic antibody research. Genetic variation in the HA gene drives both seasonal antigenic drift (mutation) and pandemic antigenic shift (reassortment), leading to recurring epidemics and pandemics, such as H1N1 and H3N2 subtypes in humans and high-pathogenic avian strains (e.g., H5N1). Targeting HA through vaccines and antibody therapies remains the principal strategy for influenza prevention and control, although rapid evolution of the protein presents major challenges for long-term efficacy[1][3][4][5][9].

Other names

HemagglutininInfluenza hemagglutininHAInfluenza A hemagglutininHx (where x denotes subtype, e.g., H1, H5)

Drugs targeting HA work via several mechanisms: Antibodies and entry inhibitors block HA-sialic acid interaction, inhibiting receptor binding. Other antibodies targeting the stalk domain can prevent membrane fusion. Furthermore, vaccines based on HA elicit protective immunity primarily through antibody responses, thereby training the immune system to target the protein.