More than 300 inherited blood group antigens have been described on the surface of human red cells. With rare exceptions, these antigens were first revealed by detection of antigen-specific antibodies present in human serum with the use of the agglutination method. Fewer than 50 of these blood group antigens are known to be polymorphic( two or more clearly different phenotypes exist in the same population of a species) in any region of the world. Out of all the antigens which are polymorphic, there are still fewer which stimulate antibodies of clinical significance by causing transfusion reactions.
Safest product for transfusion are those which are unlikely to envoke the formation of antibodies. For patients who may require long-term transfusion therapy the better approach is to check for any antibodies for one or more red cell antigens (such as K, k, Fya, Fyb, Jka, Jkb, S, s) and also an extended phenotype of all the common antigens. Thereafter, blood products which suit the entire antigen and antibody profile should be given.