Although more than 400 blood groups have been identified, the ABO blood group system remains the most impoant in clinical medicine because:
It was the first blood group system to be discovered
It has four different blood groups A,B,AB4O(H)
ABO (H) antigens are present in most body tissues and fluids.
ABO (H) antibodies are invariably present in plasma when persons RBC lacks the corresponding antigen.
D i.e. ABO antibodies are invariably present in plasma when person RBC lacks the corresponding antigen