The study of red blood cell (RBC) antigens and antibodies forms the foundation of transfusion medicine. Serologic studies initially characterized these antigens, but now the molecular composition and structure of many are known. Antigens, either carbohydrate or protein, are assigned to a blood group system based on the structure and similarity of the determinant epitopes. Other cellular blood elements and plasma proteins are also antigenic and can result in alloimmunization, the production of antibodies directed against the blood group antigens of another individual. These antibodies are called alloantibodies. Antibodies directed against RBC antigens may result from "natural" exposure, particularly.
