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*Scores are 1-8, depending on the percentage of dyed cells observed.

*Scores are 1-8, depending on the percentage of dyed cells observed.

Some epitopes are more important than others with respect to organ rejection. Therefore, certain mismatches are allowable if the critical epitopes match.

Screening

Successful organ transplant depends on minimal reaction of the recipient immune system to the antigens of the donor organ. Normal sera do not have antibodies against human antigens, termed anti-human antibodies or alloantibodies. Persons who have had a previous organ transplant, blood transfusions, or pregnancies, however, will have anti-human antibodies (termed humoral sensi-tization) that may react against a new donor organ. The chance of a successful transplant is improved by defining the specificity of the alloantibodies and selecting a suitable organ that does not have HLA antigens corresponding to the antibodies in the patient's sera.25 Humoral sensitization and the identity of alloantibodies present in the recipient serum are determined in a modified version of the CDC assay using the patient's serum as the source of antibodies and reference lymphocytes of known HLA types prevalent in the general population. The reference lymphocytes are defined by their recognition of panel reactive antibodies (PRA). This test against PRA reveals the percentage of the general population with whom the patient will cross-react.

The percentage of the panel of lymphocytes killed by the sera is referred to as %PRA. Patients with %PRA activity of more than 50% are considered to be highly sensitized; finding cross-match-negative donors is more difficult in these cases.26

Screening of sera with microparticles (beads) is performed in laboratories with flow cytometry capability (Fig. 15-7). For this method, microparticles are attached to pools of antigens derived from cell lines of defined HLA types. The microparticles are exposed to test serum, and those beads carrying antigens that are recognized by antibodies present in the test serum will bind to those antibodies. After removal of unbound antibodies, a fluo-rescently labeled secondary reporter antibody is applied, and the antibody-bound beads are detected by flow cytometry. The advantages of this method over the CDC test are that the reaction is performed in a single tube and there is less subjectivity in the interpretation of results. Because this test uses pooled antigens, however, it can detect prevalence of anti-human antibodies in the test serum but not identify which specific antibodies are pres ent. A negative result does preclude further alloantibody assessment. A variation of this method developed by Luminex Corp. and MiraiBio utilizes beads with their own internal fluorescence. By conjugating known antigens to beads of different internal fluorescence, the positively reacting antibodies can be identified while still performing the test in the same tube.

Crossmatching

The CDC test is also used for crossmatching potential organ donors and recipients. For crossmatching, recipient serum is the source of antibodies tested against donor lymphocytes (Fig. 15-8). If the recipient serum kills the donor lymphocytes, it is a positive crossmatch and contraindication for using the crossmatched donor.

Other methods used for crossmatching include variations on the lymphocytotoxicity assay and nonlymphocy-

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