Blood group homology only depends on the same ABO and Rh blood groups, but Rh blood groups have three pairs of alleles to form three antigens, Cc, Dd and ee, and there are several combinations. If the recipient is a CDE combination, and the genome combination of the donor is not exactly the same, different antigens will stimulate the body to produce corresponding antibodies, and the next time the recipient receives the corresponding antigen stimulation, there will be a blood transfusion reaction. In addition, there are subtypes of ABO blood group system, and different subtypes will have blood transfusion reaction when they stimulate each other. Usually blood stations don't test subtypes. Cross-matching is to find these possible incompatible factors.