The ABO blood group system is characterized by the presence of A and B antigens; the Rh blood group system is characterized by the absence of natural anti-Rh antibodies and the presence of incomplete antibody IgG. 1. Characteristics of ABO blood group system (1) There are two different antigens in the ABO blood group system, known as A and B antigens. The human serum contains two antibodies corresponding to them, i.e. anti-A antibody and anti-B antibody. (2) The classification of ABO blood group is based on the presence or absence of A and B antigens on the red blood cell membranes and the blood is divided into four types: those with only A antigen on the red blood cell membranes are classified as type A; those with only B antigen are classified as type B; those with both A and B antigens are classified as type AB; and those lacking both A and B antigens are classified as type O. (3) The sera of people with different blood types contain different antibodies, but they will not contain antibodies corresponding to their own red blood cell antigens, that is to say, the sera of people with blood type A contain only anti-B antibodies, the sera of people with blood type B contain only anti-A antibodies, the sera of people with blood type AB do not have anti-A and anti-B antibodies, and the sera of people with blood type O contain both anti-A and anti-B antibodies. 2. Characteristics of the Rh blood group system (1) Natural anti-Rh antibodies do not exist in human serum. Anti-Rh antibodies are produced only when Rh-negative people receive Rh-positive blood through humoral immunity. (2) When an Rh-negative recipient receives Rh-positive blood for the first time, no agglutination reaction will occur, but antibodies against the Rh antigen will be produced in his blood; when he receives another Rh-positive transfusion, an antigen-antibody reaction will occur, and the Rh-positive red blood cells that have been imported will be agglutinated and hemolyzed. Therefore, even repeated input of blood from the same donor may cause a transfusion reaction due to Rh blood group incompatibility. (3) The antibodies of the Rh system are mainly incomplete antibodies IgG, which are small molecules and can cross the placenta. When an Rh-negative woman conceives an Rh-positive fetus, the red blood cells of the fetus can enter the mother’s body in small amounts, causing the mother to produce anti-Rh antibodies. This antibody can cross the placenta into the fetus’s bloodstream, causing agglutination and hemolysis of the fetus’s red blood cells, leading to the death of the fetus.