The likelihood of tissue or organ transplant acceptance can be in- creased by using persons who have a great degree of genetic similarity. The greater the similarity, the more likely the persons will have similar MHC complexes. The reason for this is that 77 histocompatibility alleles are determined by just four human histocompatibility genes A, B, C, and D (which includes DQ, DR, and DP).
These genes are transmitted from parents to offspring according to Mendelian genetics. For example, two siblings (brother and sister) may have approximately a 25% chance of being B identical, a 50% chance of sharing one B gene, and only a 25% chance of having completely different B genes. Therefore, when tissue or organ transplants are performed, a deliberate at- tempt is made to use tissues from siblings or other genetically related, histocompatible people. Once a tissue donor has been selected, various in vitro tests are per- formed to find how closely the MHCs match. The greater the degree of similarity, the greater the chances are that the transplant will be successful. One such test is the lymphocytotoxic cross-match test. In this test the recipient’s serum is mixed with the donor’s lymphocytes to determine whether anti- bodies are present that will cause tissue rejection. The cellmediated lymphocytosis test, in which recipient leukocytes are mixed with donor serum, is used to test for acute rejection. After a prolonged incubation period, the presence of activated killer cells is an indication that rejection will occur.
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