Influenza A hemagglutinin stalk domain (HA-stalk (not a universal standard, but commonly used in scientific literature for the stalk domain of hemagglutinin))

The **Influenza A hemagglutinin stalk domain** is a highly conserved, membrane-proximal region of the hemagglutinin (HA) surface glycoprotein of the influenza A virus. Hemagglutinin is a trimeric protein that mediates viral attachment and entry into host cells. Each monomer consists of two subunits, HA1 and HA2, generated by proteolytic cleavage. The stalk domain is primarily composed of the C- and N-terminal regions of the HA1 subunit and the majority of the HA2 subunit. In contrast to the variable globular head (receptor-binding) domain, the stalk domain is highly conserved across different influenza A subtypes, largely due to its essential role in mediating the pH-triggered fusion between viral and endosomal membranes, which is necessary for viral genome release into the host cell. Antibodies that target this domain can neutralize a broad spectrum of influenza A viruses and are a focus for the development of universal influenza vaccines and therapeutic antibodies. The stalk domain is considered a structural target for broad, cross-strain protection because its function limits its ability to tolerate mutations without loss of viral fitness. Detection of anti-HA stalk antibodies is used in research as a biomarker for broadly protective immunity. Currently, there are no drugs approved that target this domain directly, but several monoclonal antibodies are under investigation.

Other names

Hemagglutinin stem domainHA stemHA stalkStalk domain of influenza A hemagglutinin

Broadly neutralizing antibodies bind to conserved epitopes in the stalk domain, inhibiting the fusion step of viral entry and thus blocking infection. These antibodies can mediate antibody-dependent cellular cytotoxicity (ADCC) and complement activation to clear infected cells.