ABO blood group, a system for classifying human blood on the basis of antigenic components of red blood cells and their corresponding antibodies. The ABO blood group is identified by the presence or absence of two different antigens, A and B, on the surface of the red blood cell. The four blood types in this grouping, A, B, AB, and O, are determined by and named for these antigens. Each ABO blood group also contains naturally occurring antibodies to the antigens it lacks. Group A has A antigens on the red cells, with anti-B antibodies in the plasma. Group B has B antigens on the red cells, and anti-A antibodies in the plasma. Group O has neither A nor B antigens, and both anti-A and anti-B in the plasma. AB has both A and B antigens on the red cells, and no anti-A or anti-B in the plasma. In addition to its significant role in transfusion therapy and transplantation, ABO blood grouping contributes to forensic medicine, to genetics, and to anthropology. See also blood group, Rh factor, transfusion.
