Physiology 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 a person 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 a person RBC lacks the corresponding antigen.