<prompt> 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 discoveredIt has four different blood groups A,B,AB4OHABO 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.  <response>: D i.e. ABO antibodies are invariably present in plasma when person RBC lacks the corresponding antigen 