Human Leukocyte Antigen HLA Testing

Introduction – HLA Testing

The human leukocyte antigen (HLA) test is also known as HLA typing or tissue typing. This test identifies antigens on the white blood cells that determine tissue compatibility for organ transplantation. There are six loci or locations on chromosome 6, where the genes that produce HLA antigens are inherited. These loci are HLA-A, HLA-B, HLA-C, HLA-DR, HLA-DQ, and HLA-DP.

Many different alleles can be inherited at each of the HLA loci. These are defined by antibodies that recognize specific HLA antigens, or by DNA probes that recognize the HLA allele. Using specific antibodies, many alleles can be recognized such as 26 HLA-A alleles, 59 HLA-B alleles, 10 HLA-C alleles, 26 HLA-D alleles, 22 HLA-DR alleles, nine HLA-DQ alleles, and six HLA-DP alleles. This high degree of genetic variability is known as polymorphism.

Significance and purpose of HLA testing:

  1. HLA typing, along with ABO (blood type) grouping, is used to test tissue compatibility. The HLA antigens expressed on the surface of the lymphocytes of the recipient are matched against those from various donors. This test is performed for transplant of kidney, bone marrow, liver, pancreas, and heart. The probability of success of a transplant increases with the number of identical HLA antigens.
  2. Graft rejection occurs when the immune cells (T-lymphocytes) of the recipient recognize specific HLA antigens on the donor’s organ as foreign and initiate a cellular immune response that attacks the graft.
  3. Sometimes, T-lymphocytes in the grafted tissue recognize the host tissues as foreign and produce a cell-mediated immune response against the recipient. This leads to a life-threatening condition called as graft versus host disease. HLA testing is performed to reduce the probability of both rejection and GVHD.
  4. Typing is also used to solve the parental dispute cases. The HLA antigens of the mother, child, and alleged father can be compared. When an HLA antigen of the child cannot be attributed to the alleged father, then the latter is excluded as the father of the child.
  5. A third use of HLA testing called linkage analysis is based on the region where the HLA loci are positioned. This region is called as the major histocompatibility complex (MHC), which contains many other genes located very close to the HLA loci. Many of the MHC-region genes are involved in immunological processes. As a result, alleles that are known to increase the chance of developing various autoimmune diseases have remained associated with specific HLA alleles. Some people having the HLAB27 allele develop an arthritic condition of the vertebrae called ankylosing spondylitis. Thus the inheritance of this condition in the family of a positive person can be determined. Some autoimmune diseases having a strong association with HLA antigens include Hashimoto’s thyroiditis HLA-DR5; Graves’ disease (HLA-B8 and Dw3); and hereditary hemochromatosis (excess iron stores), associated with HLA-A3, B7, and B14.

Preparation:

The HLA test just requires a blood sample. There is no need for the patient to fast before the test. This test should be postponed if the patient has recently undergone a transfusion, as any WBCs from the transfusion donor may interfere with the tissue typing of the patient’s lymphocytes.

Procedure:

Histocompatibility testing consists of three tests:

  1. HLA antigen typing (tissue typing)
  2. Screening of the recipient for anti-HLA antibodies (antibody screen)
  3. Lymphocyte crossmatch (compatibility test).

These tests can be performed by serological or DNA methods.In any case, HLA typing of HLA-A, HLA-B, HLA-DR, and HLADQ antigens is performed for solid organ transplants. HLA typing of HLA-C antigens is only included for bone marrow transplants.

The HLA test just requires a blood sample. There is no need for the patient to fast before the test.
The antibody screen is performed to detect antibodies in the recipient’s serum that react with HLA antigens. The method for HLA antibody screening is called as microcytotoxicity test. If an antibody against an HLA antigen is present, it will bind to the cells. The higher the number of different HLA antibodies, the lower the probability of finding a compatible match.

In the crossmatch test, the peripheral blood lymphocytes from the donor are separated into B and T lymphocyte populations. The serum from the recipient is mixed with T-cells or B-cells from the donor. A positive finding indicates the presence of preformed antibodies in the recipient that are reactive against the donor tissues. A negative T-cell crossmatch contraindicates transplantation of a tissue from the T-cell donor.

Normal results

HLA typing is reported as the phenotype for each HLA loci tested. The antibody screen test is reported as the percentage of panel reactive antibodies (PRA). The percent PRA is the number of wells reactive with the patient’s serum expressed in percent. The crossmatch is reported as compatible or incompatible.

- Link of interest: Twitter Bladder Cancer – Cancer Treatment.

 

This article is not medical advice nor a substitute to professional health advice. Always consult a doctor.

 

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