Hemagglutinin

Influenza hemagglutinin (HA) or haemagglutinin (British English) is a glycoprotein found on the surface of the influenza viruses. It is responsible for binding the virus to cells with sialic acid on the membranes, such as cells in the upper respiratory tract or erythrocytes. It is also responsible for the fusion of the viral envelope with the endosome membrane, after the pH has been reduced. The name "hemagglutinin" comes from the protein's ability to cause red blood cells (erythrocytes) to clump together ("agglutinate") in vitro.

Hemagglutinin or haemagglutinin refers to glycoproteins which cause red blood cells (RBCs) to agglutinate. This process is called hemagglutination or haemagglutination.

Hemagglutinin are the receptor-binding and membrane fusion glycoproteins of influenza viruses. They recognize sialic acid-containing, cell-surface glycoconjugates as receptors but have limited affinity for them, and, as a consequence, virus attachment to cells requires their interaction with several virus HAs. Receptor-bound virus is transferred into endosomes where membrane fusion by HAs is activated at pH between 5 and 6.5, depending on the strain of virus. Fusion activity requires extensive rearrangements in HA conformation that include extrusion of a buried "fusion peptide" to connect with the endosomal membrane, form a bridge to the virus membrane, and eventually bring both membranes close together.