How is tissue typing done

Similar to determining red cell types, it was recognized early that the best approach would be to find antibodies directed against cells other than red cells, namely white cells. The earliest attempts, therefore, used a similar agglutination reaction against white cells to that used for red cells. The first antibody to a type or specificity was found by Dausset in 1958 in sera from patients immunized by transfusions, using the leukoagglutination reaction. Patients immunized by pregnancies were also shown, by Payne, to have agglutinating antibodies to leukocytes. Because the reactions produced were so complex, van Rood introduced the use of computers to identify sera which were similar to each other. In this way, he first defined an allelic group of sera, 4a and 4b.

It was at this point in 1964 that the micro lymphocyte cytotoxicity method of Terasaki and McClelland was introduced. This method, together with some modifications, has been adopted and used universally for the past 30 years. The basic feature of requiring an extremely small volume of antisera (0.001 ml per test) and a small number of lymphocytes was critical to its widespread adoption.

In 1969, a group of investigators met at the National Institutes of Health (NIH) and agreed to use identical incubation times (30 minutes with antiserum and 60 minutes with complement) to standardize the test (often, since, called 'the NIH test').

Starting with the discovery of the molecular structure by Bjorkman in 1987, and through the subsequent establishment of the amino acid sequence of each of the HLA specificities and the development of the polymerase chain reaction (PGR) in recent years, it has become possible to determine HLA type by DNA-based methods. These methods have the advantage of not being dependent on rare, specific antisera, but rather on probes and primers which can be readily synthesized. In addition, they do not rely on living lymphocytes, as DNA can be isolated from any nonviable nucleated cell. The end-point of the tests is also not influenced by the viability of cells or the activity of complement. Perhaps most importantly, DNA typing can identify 'splits' or variants in the specificities which are impossible to detect serologically.

Currently, one drawback to the DNA tests is that they take a longer time to perform (3-6 hours). This time is further extended when all the specificities are typed. The expense is considerably higher, particularly for class I. Many of the amino acid substitutions detected by DNA typing may not be relevant for transplantation as they do not all form immunogenic epitopes against which the body reacts. When antibodies are formed against an epitope, it is clear that the target epitope is one to which the host responds immunologically. The relevance of each variant remains to be established, particularly for solid organ transplantation. For bone marrow transplants, the 'minor' mismatches appear to have some effect on the success of the transplant.

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