Hemagglutinins

antibodies capable of stimulating the agglutination of red blood cells. Hemagglutinins are subdivided into autohemagglutinins, which act on the blood cells of one’s own body, homohemagglutinins, which act on the red blood cells of organisms of the same species, and heterohemagglutinins, which act on the blood cells of other animal species. Some hemagglutinins are active only against red blood cells suspended in an isotonic sodium chloride solution, and others only in the presence of hydrophilic colloids, such as albumin, or after preliminary action on red blood cells by a proteolytic enzyme. Special hemagglutinins, called frigohemagglutinins, manifest activity only at a temperature of about 4° C.