Packed red blood cells Rh type, specifically the phenotype, and the "screen" refers to testing for atypical antibodies that might cause transfusion problems. The typing and screening are also performed on donor blood. The blood groups represent antigens on the surface of the red blood cells which might react with antibodies in the recipient. The ABO blood group system has four basic phenotypes: O, A, B, and AB. In the former Soviet Union these were called I, II, III, and IV, respectively. There are two important antigens in the system: A and B. Red cells without A or B are called type O, and
Blood type (non-human) S, and Z polymorphisms. Blood type (non-human) Animal erythrocytes have cell surface antigens that undergo polymorphism and give rise to blood types. Antigens from the human ABO blood group system are also found in apes and Old World monkeys, and the system traces back to the origin of hominoids. Other animal blood sometimes agglutinates (to varying levels of intensity) with human blood group reagents, but the structure of the blood group antigens in animals is not always identical to those typically found in humans. The classification of most animal blood groups therefore uses different blood typing systems to those used