Hemagglutinin

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In molecular biology, hemagglutinins (alternatively spelt haemagglutinin, from the Greek haima, 'blood' + Latin gluten, 'glue') are receptor-binding membrane fusion glycoproteins produced by viruses in the Paramyxoviridae and Orthomyxoviridae families.^[1][2] Hemagglutinins are responsible for binding to receptors on red blood cells to initiate viral attachment and infection.^[3] The agglutination of red cells occurs when antibodies on one cell bind to those on others, causing amorphous aggregates of clumped cells.

Hemagglutinins recognize cell-surface glycoconjugates containing sialic acid on the surface of host red blood cells with a low affinity and use them to enter the endosome of host cells.^[4] In the endosome, hemagglutinins are activated at a pH of 5–6.5 to undergo conformational changes that enable viral attachment through a fusion peptide.^[5]

Virologist George K. Hirst discovered agglutination and hemagglutinins in 1941.^[6] Alfred Gottschalk proved in 1957 that hemagglutinins bind a virus to a host cell by attaching to sialic acids on carbohydrate side chains of cell-membrane glycoproteins and glycolipids.^[7]