Hemagglutinin

Hemagglutinin is a complex surface protein which binds virus to sialic acid-containing glycoproteins and glycolipids on host cell surfaces. Antibody to hemagglutinin neutralizes viral infectivity by blocking binding and uptake by host cells.

Neutralizing anti-hemagglutinin antibodies occur as a result of natural infection or influenza immunization. The presence of these antibodies protects against infection by viruses carrying the same hemagglutinin subtype. Three subtypes of influenza hemagglutinin have arisen by major antigenic shifts (i.e., H1, H2, H3); they are poorly cross-protective. There are also a vast number of variants of each subtype which have arisen by minor antigenic drift; these variants are variably cross-protective within a subtype.