적혈구
Agglutination, using blood agglutinins known as hemagglutinins, is used diagnostically to identify blood types of human beings based on the reaction between the erythrocyte (Red blood cell) antigens and agglutinins. Human erythrocytes have two main types of antigens (Antigen A and B) expressed in different combinations to give either erythrocytes that express only antigen A, antigen B, antigen A and B together or no antigen at all. When erythrocytes are exposed to hemagglutinins (anti-A and Anti-B antibodies), those expressing antigen A or B coagulate upon contacting anti-A and anti-B hemagglutinins respectively. Erythrocytes expressing both antigens coagulate upon contacting either anti-A or anti-B hemagglutinins while those not expressing any antigen do not coagulate upon contact with any hemagglutinin. This technique serves as a quick and effective method for identifying the blood type of a person and it is essential when blood transfusion has to be performed quickly to replenish lost blood.