Although more than 400 blood groups have been identified, the ABO blood group system remains the most important in clinical medicine because -
The basic difference between the ABO blood group system and other blood group systems (such as Rh, Kell, Duffy, MNSs blood groups), which makes the ABO group so important is that -

Preformed ABO antibodies (agglutinins) are present in person serum when his RBCs lack the corresponding antigen (ie. anti B antibody would be present in a person of type A and type O blood groups. These two blood groups do not have the 'B' antigen on the RBCs) 


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, an 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 mismatched)
These preformed antibodies, rapidly destroy the RBCs of any mismatched blood transfusion.
Read the following lines from the Journal of hematology-
"In clinical transfusion practice, the ABO blood groups are the most important and can never be ignored in red cell transfusion, because individuals 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".
It was the first blood group system to be discovered -
It has four different blood groups A,B, AB, O (H).
ABO(H) antigens are present in most body tissues and fluids
ABO(H) antibodies are invariably present in plasma when person's RBC lacks the corresponding antigen