<prompt> Although more than 400 blood groups have been identified, the ABO blood group system remians the most impoant in clinical medicne because ?It was the first blood group system to be discovered -It has four different blood groups A,B, AB, 0 H.ABOH antigens are present in most body tissues and fluidsABOH antibodies are invariably present in plasma when persons RBC lacks the corresponding antigen  <response>: Ans. is d i.e., ABO H antibodies are invariably present in plasma when persons RBC lacks the corresponding antigeno The basic difference between ABO blood group system and other blood group systems such as Rh, Kell, Duffy,MNSs blood groups, which makes the ABO group so impoant is that ?Preformed ABO antibodies agglutinins are present in persons serum when his RBCs lack the corresponding antigen ie. anti B antibody would be present in a person of type A and type 0 blood groups. These two blood groups do not have the B antigen on the RBCso This is not the case with other blood groups. Preformed antibodies are not present. They are formed only after an exposure to the antigen, for example Rh negative person do not have anti- Rh antibodies, these antibodies are formed only after an exposure to Rh positive blood by a blood transfusion, i.e. this makes the first blood transfusion safe even if mismathcedo These preformed antibodies, rapidly destroy the RBCs of any mismatched blood transfusion.o Read the following lines from the Journal of hematology?In clinical transfusion practice, the ABO blood groups are the most impoant and can never be ignored in red cell transfusion, because individials who genetically lack any antigen, have antibodies against the red cell types that they have not inherited. These antibodies can destroy red cells rapidly in circulation. 