HLA Testing

The primary use for human leukocyte antigen (HLA) testing is to match organ and tissue transplant recipients with compatible donors. It identifies the major HLA genes a person has inherited and the corresponding antigens (proteins) that are present on the surface of their cells. These antigens help the body's immune system distinguish which cells are "self" and which are "foreign" or "non-self." Any cells that are recognised as "non-self" can trigger an immune response, including the production of antibodies.

HLA testing also includes screening transplant recipients for the presence of antibodies that might target the donated tissue or organ as part of an immune response.

HLA testing is important in medicine when transplanting tissue or an organ(s). In bone marrow transplants, for instance, the HLA genes and antigens 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 to not be attacked or rejected by the recipient's immune system.

Different kinds of transplants necessitate different levels of matching between donor and intended recipient. This may determine which HLA tests are performed and which HLA genes are tested for.

There are typically three components of HLA testing used to determine compatibility:

• HLA typing of donors and recipients - this step involves identifying HLA alleles. It may involve serological HLA testing or molecular (DNA) typing.

Family members who volunteer to donate bone marrow or an organ are HLA tested to see if they are a match for the relative who needs a transplant.

If people want to make themselves available to donate bone marrow to anyone who needs it, they can sign up with a national registry such as the Anthony Nolan Bone Marrow Registry.  HLA testing is performed and test results are kept on file to compare with results from those people who are seeking a match.

• HLA antibody screening of recipients - HLA antibody testing is performed on the recipient to determine if there are any antibodies present that would target the donated organ or tissue. Some people have HLA-specific antibodies that have developed following an exposure to non-self antigens. There are essentially three reasons for exposure to non-self HLA: pregnancy, particularly multiple pregnancies (from exposure to the father's HLA that have been passed on to the foetus), blood or platelet transfusions, or previous organ transplant(s). Once present, HLA antibodies must be considered during matching because they will potentially attack donor tissues that have the corresponding HLA type.

HLA antibody testing may be periodically performed and updated to determine if the person waiting for a matching organ to become available has developed additional HLA antibodies. HLA antibody assessment can also be used post-transplant to determine if the recipient has developed new or increased levels of antibodies to the donor.

• Lymphocyte crossmatching (Donor-specific) - This step occurs after a potential donor has been identified. It helps determine if the intended recipient has antibodies directed against antigens present on the donor's lymphocytes. Serum from the intended recipient is mixed with white blood cells (T and B lymphocytes) from the donor. Any reaction detected (a positive result) would indicate likely incompatibility between the two. The crossmatch result should always be interpreted along with known information regarding the recipient's HLA antibodies and the donor's HLA typing.

Sometimes HLA gene testing is used to aid in the diagnosis of an autoimmune disease. The HLA system is one of the mechanisms involved in the body's recognition of "self" and "non-self" antigens and the immune response to foreign substances. The body can inappropriately produce an immune response against its own cells, producing antibodies (autoantibodies). Certain disease states have been found to be associated with particular HLA antigens, such as ankylosing spondylitis which is associated with HLA-B27. HLA typing may also be important in preventing reactions to certain medications as side effects have been noted with certain drugs and specific HLA types. (For more on this, see Common Questions #2).

Transplant recipients: HLA gene or antigen and antibody testing is typically performed when it is first determined that a person needs to have an organ or bone marrow transplant. HLA gene and antigen results will not change over time, but HLA antibody testing may be done periodically and after events, such as a pregnancy or a blood transfusion, to see if the potential recipient has developed additional HLA antibodies. Sometimes HLA antibodies develop in those who receive frequent blood or platelet transfusions. Identification and periodic monitoring of the antibodies may be necessary.

HLA antibody assessment is also used post-transplant to determine if the recipient has developed new or increased levels of antibodies to the donor's transplanted organ. The presence of antibodies to donor antigens, along with other assessments such as a tissue biopsy sample of the transplanted organ, may indicate that the recipient is experiencing rejection of the transplanted organ. This information is important for the healthcare professional to assess and treat the rejection.

Transplant donors: HLA gene or antigen typing is performed for family members when they have volunteered to see if they are a match for a relative who needs a kidney, liver, bone marrow, or other type of transplant. Living unrelated persons may also be tested as potential donors for organ transplant, and this is increasingly the case for kidney transplants. HLA typing is also performed on unrelated individuals who wish to become a bone marrow donor through the donation registry.

When an organ is from a deceased donor, HLA testing is performed to match it as quickly as possible to a potential recipient or recipients. The amount of time available to ensure the greatest viability of the organs or tissues ranges from a few hours to no more than a day or two.

Crossmatch testing is done after a potential donor has been identified through HLA typing. This test is typically performed just prior to an organ transplant to ensure that there is no mismatch. In the case of living donor transplantation, the crossmatch compatibility is usually performed more than once, when the donor is initially identified and again just before the actual transplant procedure.

Specific HLA genes or antigens are identified during HLA typing for organ and tissue transplant compatibility. The genes and/or antigens of transplant recipients are compared to those of potential donors. Results indicate how many antigens match and how many mismatches are present. The greater the number of matches the more likely the transplant will succeed. "0 mismatches" indicates a high probability that the organ or tissue will not be rejected by the recipient.

The absence of recipient HLA antibodies to the donor HLA antigens is very important. Matching a donor with a recipient who has developed antibodies must be carefully considered because the more HLA antibodies a person has developed, the higher the probability for rejection.

A positive (reactive) crossmatch result is usually interpreted as a high risk transplant. These people are at risk for rejection of the transplant, which may or may not be treatable with various immunosuppressant drugs.

HLA testing is performed in laboratories that specialise in histocompatibility and immunogenetics and are accredited by the United Kingdom Accreditation Scheme (UKAS) and the European Federation of Immunogenetics (EFI).

Histocompatibility involves testing donors and recipients to see if they are HLA matches. Immunogenetics is the study of the relationships between the immune system, genetics, and disease development.

Historically, HLA testing was used to help identify someone (forensic testing) or to determine if people were related (parentage testing), although now there are other, more specific molecular tests available for these purposes.