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HLA Testing

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Also known as: Tissue Typing; HLA Typing; Histocompatibility Testing; HLA Crossmatching; HLA Antibody Testing/Screening/Identification
Formal name: Human Leukocyte Antigen; HLA Oligotyping; HLA Sequence-based Typing
Related tests: Blood Typing; HLA-B27; HLA-A, B, C, DR and DQ

The Test Sample

What is being tested?

Human leukocyte antigens (HLA) are part of the major histocompatibility complex or MHC. They refer to certain specialized proteins (antigens) present on the surface of all nucleated cells in the body and the genes that code for them. Everyone has an inherited combination of HLA antigens present on the surface of his or her white blood cells (leukocytes) and other cells that contain a nucleus. HLA testing identifies the major HLA genes a person has inherited and their corresponding antigens that are present on the surface of their cells.

HLA antigens and the MHC system play an important role in the management of the immune system. They help the body's immune system distinguish which cells are "self" and which are "foreign" or "non-self." Any cells that are recognized as "non-self" can trigger an immune response, including the production of antibodies. HLA antibody testing is also performed on transplant recipients to determine if there are any antibodies present that would target the donated organ or tissue.

This is important in medicine when transplanting tissue or an organ(s). In bone marrow transplants, the HLA genes that the donor and recipient have need to be the same or match as closely as possible for a transplant to be successful and for the tissue not be attacked or rejected by the recipient's immune system. Also, bone marrow donors and recipients must match closely so that the immune cells (lymphocytes) in the donated bone marrow do not attack the recipient's cells in a process called graft-versus-host disease (GVHD; see Common Questions #4).

In solid organ transplants, such as kidney or lung transplants, it is ideal to match the antigens between the donor and recipient; however, the typing incompatibilities are less critical as long as the recipient has not produced antibodies directed against donor's antigens. Various drugs may be administered to help suppress the recipient’s immune system in order to minimize organ rejection.

When the donor organ is compatible with the intended recipient, it is less likely to be rejected in the immediate post-transplant period. HLA testing, along with ABO blood typing, is used to identify and match organ and tissue transplant donors with recipients who have the same or an acceptable number of matching HLA genes and antigens.

Finding a donor who is compatible with an intended recipient may sometimes be difficult. Part of the reason is because each particular HLA gene can have numerous possible forms or variations (alleles). This is referred to as polymorphic. In addition, there are more than 200 genes that make up the large "gene family" of the HLA system. With many different possible combinations and numerous HLA genes to take into account, it can be a challenge to find a suitable donor.

However, HLA genes that are located close together, as seen on chromosome 6, are inherited together as groups known as haplotypes; thus, a child inherits one haplotype from each parent. Because of this, there is a greater chance that family members will have the same group of HLA genes compared with non-related potential donors. Often, a recipient's parents, children, or siblings may serve as the best transplant matches.

Example of inheritance of HLA haplotypes

Read the article on The Universe of Genetic Testing for more on inheritance and the HLA system.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm. Sometimes, for HLA typing, a swab of cells is collected from the inside of the cheek (a buccal swab).

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.