Blood Grouping: Injury or surgery can lead to a blood transfusion. Transfusion reactions/Agglutination: clumping of blood cells (bad). Antigens: molecules on surface of erythrocytes. Antibodies: proteins in plasma that bind to specific antigens. Blood groups: named according to antigen (ABO). ABO Blood Groups • In the ABO blood group system, there are two types of antigens that may appear on the surface of the red blood cells, type A antigen and type B antigen. • Type A blood has type A antigens, type B blood has type B antigens, and type AB blood has both types of antigens. • Type O blood has neither A nor B antigens. • The types of antigens found on the surface of the red blood cells are genetically determined. • Antibodies against the antigens are usually present in the plasma of blood. • Plasma from type A blood contains anti-B antibodies, which act against type B antigens; plasma from type B blood contains anti-A antibodies, which act against type A antigens. • Type AB blood plasma has neither type of antibody, and type O blood plasma has both anti-A and anti-B antibodies. • In Caucasians in the United States, the distribution is type O, 47%; type A, 41%; type B, 9%; and type AB, 3%. • Among African-Americans, the distribution is type O, 46%; type A, 27%; type B, 20%; and type AB, 7%.