The most commonly used blood group systems in clinical practice are the ABO blood group system and the Rh blood group system. The ABO blood group system classifies human blood groups into A, B, O, and AB, and the differences between these four blood groups are: A blood group refers to red blood cell membrane surface expressing A antigen, and its serum does not express B antibody. Type AB blood type expresses both A and B antigens and has no antibodies in the serum. If the surface of the red blood cell membrane expresses D antigen, the blood is Rh-positive, and if the surface of the red blood cell membrane does not express D antigen, the blood is Rh-negative.