Tissue Typing

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Definition

Tissue typing is a group of procedures that determines the type of histocompatibility antigens on a person's cells or tissues. This procedure is typically used prior to transplantation of tissues or organs.

Description

Generally, typing is performed on blood cells because they are an easy sample to obtain. Blood is withdrawn from a vein in the forearm, and the cells are separated. 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 the antigen is present, the antibody will bind to it. Laboratory instruments are used 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 new DNA-based test that can detect the presence or absence of antigens by determining whether cells have the genes for the antigens.

One type of transplant does not require tissue typing. In the case of corneal transplants, tissue typing is not needed because corneas 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.

— John T. Lohr, PhD

(medicine) A procedure involving a test or a series of tests to determine the compatibility of tissues from a prospective donor and a recipient prior to transplantation.

A procedure involving a test or a series of tests to determine the compatibility of tissues from a prospective donor and a recipient prior to transplantation. The immunological response of a recipient to a transplant from a donor is directed against many cell-surface histocompatibility antigens controlled by genes at many different loci. However, one of these loci, the major histocompatibility complex (MHC), controls antigens that evoke the strongest immunological response. The human MHC is known as the HLA system, which stands for the first (A) human leukocyte blood group system discovered. See also Cellular immunology; Histocompatibility.

The success of transplantation is greatly dependent on the degree of histocompatibility (identity) between the donor and recipient, which is determined by the HLA complex. When the donor and recipient have a low degree of histocompatibility, the organ is said to be mismatched, and the recipient mounts an immune response against the donor antigen. By laboratory testing, the degree of antigenic similarity between the donor and the recipient and the degree of preexisting recipient sensitization to donor antigens (and therefore preformed antibodies) can be determined. This is known as cross-matching.

Phenotyping of HLA-A, -B, and -C (ABC typing) of an individual is determined by reacting that individual's lymphocytes with a large panel of antisera directed against specific HLA antigens. The procedure is known as complement-mediated cytotoxicity assay. The person's lymphocytes are incubated with the different antisera and complement is added. Killing of the cells being tested indicates that they express the HLA antigens recognized by the particular antiserum being used. Killing of potential donor lymphocytes in the complement-mediated cytotoxicity assay is a contraindication to transplantation of tissue from that donor. See also Complement; Hypersensitivity; Immunoassay.

In addition to its important role in organ transplantation, determination of the HLA phenotype is useful in paternity testing, forensic medicine, and the investigation of HLA-disease associations. See also Transplantation biology.