The four major blood types generally refer to the ABO blood types of red blood cells, including the four types of A, B, AB and O. 1. Blood type A: there are only A-type antigens on the red cell membrane, but no B-type antigens; there are no anti-A antibodies in the serum, only anti-B antibodies. 2. Blood type B: there are B-type antigens on the red cell membrane and no A-type antigens; there are anti-A antibodies in the serum and no anti-B antibodies. 3. Blood type AB: there are A and B antigens on the surface of the red blood cell membrane; the serum does not contain anti-A and anti-B antibodies. 4. Blood type O: There are no A or B antigens on the surface of the red cell membrane; the serum contains both anti-A and anti-B antibodies. The combination of antibodies and antigens of the same type will lead to hemolytic reaction. Therefore, blood transfusion is best done by homotypic transfusion, and cross-matching test should also be performed before transfusion.