ABO blood group system
The most important of blood-typing systems, the ABO blood group is the determinant for transfusion reactions and organ transplantation. Unlike the other blood-typing systems, the ABO blood types have far-ranging significance other than transfusion or transplantation, including the determination of many of the digestive and immunologic characteristics of the body. The ABO blood group is comprised of four blood types: O, A, B and AB. Type O has no true antigen, but carries antibodies to both A and B blood. Type A and type B carry the antigen named for their blood type and make antibodies to each other. Type AB does not manufacture any antibodies to other blood types because it has both A and B antigens. Anthropologists use the ABO blood types extensively as a guide to the development of early peoples. Many diseases, especially digestive disorders, cancer, and infection, express preferences, choosing between the ABO blood types. These expressions are not generally understood or appreciated by either physicians or the general population.