What is crossmatching? Why is cross-matching necessary before kidney transplantation? Cross-matching is a cross-cytotoxicity test, which is a complement dependent cytotoxicity (CDC) test in which the recipient’s serum is incubated with the peripheral blood lymphocytes of the donor providing the transplanted organ. If antibodies are present in the recipient’s serum, the donor lymphocytes can be killed and the crossmatch test result is positive, and the antibody strength is judged by the percentage of dead cells; if there are no anti-HLA antibodies in the recipient’s serum, the donor lymphocytes can remain viable and the crossmatch test result is negative. If there is no anti-donor HLA antibody in the recipient’s serum, the donor lymphocytes will remain viable and the cross-matching test result will be negative. If the kidney transplantation is performed, hyper-acute rejection will occur and the color of the graft will gradually change from bright red to dark red and cyanotic in a few minutes after the operation, the texture will gradually become soft and lose the feeling of fullness, and the function of the transplanted kidney will be completely lost, so the transplanted kidney must be removed immediately and the operation will be declared a failure. Otherwise, it will endanger the life of the recipient. Therefore, cross-matching test must be performed before kidney transplantation, and only when the cross-matching test result is negative, that is, there is no anti-HLA-specific antibody in the recipient’s serum, kidney transplantation can be performed to avoid the occurrence of hyperacute rejection and ensure the success of the transplantation.