According to the presence of A and B antigens on the red blood cell membranes, there are four types of blood groups in the human blood system: blood group A refers to the presence of A antigens on the red blood cell membranes and the presence of anti-B antibodies in the serum; blood group B refers to the presence of B antigens on the red blood cell membranes and the presence of anti-A antibodies in the serum; blood group O refers to the absence of both A and B antigens on the red blood cell membranes and the presence of both anti-A and anti-B antibodies in the serum; and blood group AB refers to the presence of both A and B antigens on the red blood cell membranes and the absence of both anti-A and anti-B antibodies in the serum. Type O blood refers to the absence of both A and B antigens on the red blood cell membrane, and the serum contains anti-A and anti-B antibodies. Therefore, blood type AB is also known as the universal recipient, but the amount of blood transfusion should not be too large, and should only be used in emergency situations, so the safety factor is not high.