Although more than 400 blood groups have been identified, the ABO blood group system remains the most important in clinical medicine because -

• A. ABO(H) antigens are present in most body tissues and fluids.
• B. ABO(H) antibodies are invariably present in plasma when persons RBC lacks the corresponding antigen
• C. It was the first blood group system to be discovered
• D. It has four different blood group A,B,AB,O(H)