Tissue typing

Tissue typing is a group of procedures that determines the type of histocompatibility antigens on a person's cells or tissues. This procedure is typicallyused prior to transplantation of tissues or organs to ensure as close a match as possible between the donor and the recipient. If the histocompatibilityantigens do not match well, there is a much greater chance that the recipientwill reject the donated tissue.

Histocompatibility antigens are molecules on the surface of all cells in thebody. The specific types of histocompatibility antigens present on a person'scells determine their identity and distinguish each person. They are a "fingerprint," as each person has a unique set of histocompatibility antigens. Ifthe antigens on tissue or organs from a donor do not match that of the recipient, a rejection response can occur. The recipient's immune system will detect the difference between the two sets of antigen and start a rejectionresponse to kill the donated tissue. Except in the case of identical twins,no two people are identical in terms of their histocompatibility antigen types. However, the closer two tissues come to matching, the more likely the recipient will accept the donated tissue or organ.

Human Lymphocyte Antigens (HLA) is the name given to the most commonly used histocompatibility antigens. The antigens can be grouped into two classes: class I antigens are found on almost all cells, and class II antigens are normally found only on B lymphocytes, macrophages, monocytes, dendritic cells, andendothelial cells.

Generally, typing is performed on blood cells because they are an easy sampleto obtain. Blood is withdrawn from a vein in the forearm, and the cells areseparated. There are a number of different techniques used to identify the antigens on the cells. Typically, specific antibodies react with the cells. Each antibody preparation is specific for one histocompatibility antigen. If theantigen is present, the antibody will bind to it. Laboratory instruments areused to detect antibody binding to the cells. Class II antigens are determined by the mixed lymphocyte reaction (MLR) or by a polymerase chain reaction (PCR). In the mixed lymphocyte reaction, lymphocyte replication occurs if there is a mismatch, and is detected by a specific assay. The PCR test is a DNA-based test that can detect the presence or absence of antigens by determiningwhether cells have the genes for the antigens.

One type of transplant does not require tissue typing. In the case of cornealtransplants, tissue typing is not needed because cornea do not have their own blood supply. This greatly reduces the chance that immune cells will come in contact with the cornea and recognize it as foreign. For this reason, corneas can be transplanted from any person, and there is little chance of rejection.

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