Influenza B virus hemagglutinin antigen (HA (for Hemagglutinin))

The Influenza B virus hemagglutinin antigen is a trimeric glycoprotein found on the surface of influenza B viruses. It plays a critical role in viral infectivity by mediating two essential steps in infection: 1. Attachment: The globular head domain binds specifically to sialic acid-containing receptors on respiratory epithelial cells, determining host specificity. 2. Membrane Fusion: After endocytosis into host cells, acidic pH triggers conformational changes in HA that drive fusion between the viral envelope and endosomal membrane, allowing release of viral RNA into the cytoplasm. Hemagglutinin is also responsible for agglutinative properties observed with red blood cells—a feature exploited diagnostically using assays like HI tests. The molecule contains several major antigenic sites (notably within loops 120, 150, 160, and helix 190), which are targets for neutralizing antibodies generated through natural infection or vaccination. These regions undergo frequent mutations (antigenic drift), challenging long-term immunity. As a principal target of humoral immunity—especially neutralizing antibodies—hemaglutinnin underpins current seasonal flu vaccines' protective effects against circulating strains. While no direct small-molecule inhibitors exist clinically against this protein itself (unlike neuraminidase inhibitors), experimental monoclonal antibody therapies are being explored. In summary, influenza B virus hemagluttinin is an essential therapeutic target due to its indispensable roles in both initiating infection and serving as a key immunogen driving protective antibody responses.

Other names

HemagglutininHAInfluenza B hemagglutininInfluenza B virus HA protein

Neutralizing antibodies bind to the globular head or stalk region of HA, blocking receptor binding or preventing conformational changes required for membrane fusion Vaccines induce antibody responses that inhibit hemagglutination and block infection