In the clinic, if there are patients who need to be transfused with red blood cells, cross-matching has to be done before blood transfusion. The so-called cross-matching, first of all, is to check whether the red blood cells of the blood donor and the serum of the blood recipient will undergo agglutination reaction, also known as the primary test. In addition to the primary test, it is also necessary to check whether the red blood cells of the transfusion recipient and the blood serum of the blood donor will undergo agglutination reaction, also known as the secondary test. Only if there is no agglutination reaction in both the primary test and the secondary test, the red blood cells can be transfused into the patient safely, and that is the cross-matching. For example, if type A blood is transfused to a patient with type A blood, it is necessary to check whether the red blood cells of the donor’s type A blood, and the serum of the type A blood have an agglutination reaction, and at the same time, it is also necessary to check whether the red blood cells of the recipient’s type A blood, and the serum of the donor have an agglutination reaction, and it is only safe for the transfusion if there is no agglutination reaction in both.