blood group, the classification of blood based on the presence or absence of genetically determined antigens on the surface of the red cell. Many blood group systems have been described, including ABO, Rh, MNS, P, Kell, Duffy, Kidd, Lutheran, Kx, H, Xg, and others, as well as collections of high- and low-frequency antigens. Their relative importance depends on their clinical significance in transfusion therapy, organ transplantation, maternal-fetal compatibility, and genetic studies. See also ABO blood group.
