Hemagglutinin glycoprotein of the influenza virus (HA)

Hemagglutinin glycoprotein of the influenza virus is a surface-exposed, trimeric, class I fusion protein essential for viral infectivity. It binds to sialic acid–containing receptors on host cell surfaces, facilitating viral attachment, and, after endocytosis and exposure to acidic pH, undergoes major conformational changes to mediate fusion between the viral and endosomal membranes, allowing entry of viral RNA into the host cell cytoplasm[1][3][7]. It is the principal antigenic determinant of the virus and is targeted by most neutralizing antibodies[2][5][7]. HA exists in multiple subtypes (e.g., H1, H3, H5), which, through antigenic variation, impact viral host range, pathogenicity, and susceptibility to immune responses or vaccines. Its biological properties and immunogenicity make it the main target for seasonal and pandemic influenza vaccines, monoclonal antibodies, and experimental fusion inhibitors, as well as a key biomarker in diagnostics and surveillance[1][3][7]. Mutation and reassortment of the HA gene are central to influenza’s ability to evade immunity and drive epidemics and pandemics.

Other names

HemagglutininHAInfluenza hemagglutininInfluenza virus hemagglutinin

Inhibition of HA-mediated fusion (eg. arbidol binds to HA and blocks structural rearrangement required for membrane fusion); Neutralization by antibodies (antibodies bind epitopes on the HA head or stalk, preventing receptor binding and/or conformational changes for fusion); Vaccination-induced immunity (exposure to HA antigen elicits protective antibody response)