classification of blood in terms of distinctive inherited characteristics that are associated with the antigens (antigen) located on the surface of red blood cells (erythrocytes (erythrocyte)). The ABO (ABO blood group system) and the Rh blood groups (Rh blood group system) are among those most commonly considered. Identification of these determinants has become indispensable in connection with blood transfusion, because the recipient and donor must have the same, or compatible, blood groups. Otherwise, hemolysis (destruction) or coagulation (clotting) results from interaction of an antigen on the red blood cells of one with an antibody in the serum of the other. In addition, blood typing serves to promptly identify the cause of such disorders as erythroblastosis fetalis (hemolytic disease of the newborn), which results from blood group incompatibility between mother and fetus. Since blood group determinants are inherited according to generally known mechanisms of heredity, blood typing sometimes provides a method for resolving cases of disputed paternity.