hemagglutinin

hemagglutinin, any of a group of naturally occurring glycoproteins that cause red blood cells (erythrocytes) to agglutinate, or clump together. These substances are found in plants, invertebrates, and certain microorganisms. Among the best-characterized hemagglutinins are those that occur as surface antigens (foreign proteins that stimulate the production of antibodies) on viruses in the family Orthomyxoviridae, which contains the influenza viruses, and the family Paramyxoviridae, which contains a number of pathogenic viruses, including those that cause...