A substance, usually a protein on the surface of a virus, that causes red blood cells to clump together.
From haema + agglutinin (a protein that causes agglutination), derived from Latin agglutinare. Named in the early 1900s when researchers discovered proteins on virus surfaces caused this reaction.
The 'H' in 'H1N1 flu' stands for haemagglutinin—it's literally the spike protein that the flu virus uses to grab onto your cells! Scientists track which versions of haemagglutinin are circulating each year to design new vaccines.
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