Pathology Although more than 400 blood groups have been identified, the ABO blood group system remains the most important in clinical medicine because – A It was the first blood group system to be discovered - B It has four different blood groups A,B, AB, O (H). C ABO(H) antigens are present in most body tissues and fluids D ABO(H) antibodies are invariably present in plasma when person's RBC lacks the corresponding antigen 💡 Explanation Although more than 400 blood groups have been identified, the ABO blood group system remains the most important in clinical medicine because - ✓ Correct Answer: D. ABO(H) antibodies are invariably present in plasma when person's RBC lacks the corresponding antigen