hemagglutinin

thumb|300px|Illustration showing influenza virus attaching to [[cell membrane via the surface protein hemagglutinin]] The term hemagglutinin (alternatively spelt haemagglutinin, from the Greek , 'blood' + Latin , 'glue') refers to any protein that can cause red blood cells (erythrocytes) to clump together ("agglutinate") in vitro. 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 hemagglutinins 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.

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. Virologist George K. Hirst is also credited for "discovering agglutination and hemagglutinin" in 1941. 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.