Hemagglutinin

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The term hemagglutinin (alternatively spelt haemagglutinin, from the Greek haima, 'blood' + Latin gluten, 'glue') refers to any protein that can cause red blood cells (erythrocytes) to clump together ("agglutinate") in vitro.^[1] They do this by binding to the sugar residues on a red blood cell; when a single hemagglutinin molecule binds sugars from multiple red blood cells, it "glues" these cells together. As a result, they are carbohydrate-binding proteins (lectins). The ability to bind red blood cell sugars have independently appeared several times, and as a result hemaglutinins do not all bind using the same mechanism. The ability to bind red blood sugars is also not necessarily related to the in vivo function of the protein.^[2]

The term hemagglutinin is most commonly applied to plant and viral lectins. Natural proteins that clump together red blood cells were known since the turn of the 19th century.^[2] Virologist George K. Hirst is also credited for "discovering agglutination and hemagglutinin" in 1941.^[3] 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.^[4]