HLA typing is a blood test that identifies the specific set of proteins on the surface of your cells that your immune system uses to tell the difference between your own tissue and foreign invaders. These proteins, called human leukocyte antigens, vary enormously from person to person. Knowing your HLA type is essential for matching organ and stem cell donors, diagnosing certain autoimmune diseases, and screening for dangerous drug reactions.
How HLA Proteins Work
Every nucleated cell in your body displays HLA proteins on its surface, like a molecular ID badge. Your immune system’s T cells constantly scan these badges. When they recognize your own HLA proteins carrying normal cell fragments, they move on. When they spot something unfamiliar, such as a virus fragment or a foreign cell, they launch an attack.
The genes that code for these proteins sit on chromosome 6 and form the most genetically diverse cluster in the entire human genome. This diversity is why finding a transplant match can be so difficult: the odds that any two unrelated people share the same HLA profile are extremely low. You inherit one set of HLA genes from each parent, which means any biological sibling has roughly a 25% chance of being a full match.
Class I and Class II Antigens
HLA proteins fall into two main groups. Class I proteins (labeled HLA-A, HLA-B, and HLA-C) appear on nearly all cells and alert killer T cells to infections happening inside cells. Class II proteins (HLA-DR, HLA-DQ, and HLA-DP) appear mainly on immune cells and help coordinate the broader immune response by communicating with helper T cells.
Both classes matter for transplant matching, but Class I and the DR locus from Class II carry the most weight in determining whether a donor is compatible.
Why HLA Typing Matters for Transplants
For bone marrow and stem cell transplants, the standard goal is an 8/8 match, meaning both copies of four key genes (HLA-A, HLA-B, HLA-C, and HLA-DRB1) are identical between donor and recipient. Joint guidelines from the National Marrow Donor Program and the Center for International Blood and Marrow Transplant Research recommend this 8/8 match because it leads to better survival rates, lower transplant-related mortality, and reduced risk of graft-versus-host disease, a serious complication where donated cells attack the recipient’s body. A 7/8 match can be considered when no perfect match is available.
Some transplant centers now push further, testing additional loci to achieve a 10/10 or even 12/12 match. Mismatches at the HLA-DQ locus appear to add risk, though less dramatically than mismatches at the core four. For solid organ transplants like kidneys, a smaller subset of HLA markers (typically HLA-A, HLA-B, and HLA-DR) is typed before a patient can be placed on the waiting list.
HLA Typing and Disease Risk
Certain HLA variants are strongly linked to autoimmune and inflammatory diseases. The connection doesn’t mean you’ll definitely develop the condition, but it significantly raises your risk.
- Ankylosing spondylitis: About 85% of patients with this inflammatory spine condition carry the HLA-B27 allele. Testing for B27 is one of the tools doctors use alongside imaging and symptoms to support a diagnosis.
- Celiac disease: Roughly 90% of people with celiac disease carry the HLA-DQ2 gene variant, and another 5% carry HLA-DQ8. One study in Spain found that 98% of celiac patients were positive for one or both variants, compared to about 49% of healthy controls. Importantly, carrying these genes doesn’t guarantee celiac disease. Over half of the general population carries DQ2 or DQ8 without ever developing it. A negative HLA test, however, is useful because it essentially rules celiac disease out.
Screening for Drug Reactions
HLA typing has a growing role in pharmacogenomics, the practice of using genetic information to predict how someone will respond to a medication. The clearest example involves abacavir, an HIV drug. People who carry the HLA-B*5701 allele face a significant risk of a dangerous hypersensitivity reaction to this medication. In a landmark clinical trial, screening patients before prescribing abacavir completely eliminated confirmed hypersensitivity reactions: 0% in the screened group versus 2.7% in the unscreened group. That screening test is now standard practice before starting abacavir, and it demonstrated that a simple genetic test can prevent a specific drug toxicity.
How the Test Is Done
HLA typing requires only a blood sample or a cheek swab. The lab extracts your DNA and analyzes the specific HLA gene sequences. Results come in two levels of detail. Low-resolution typing identifies broad antigen groups, which is sometimes sufficient for initial screening or disease association testing. High-resolution typing pinpoints the exact allele variants you carry, and this level of precision is critical for stem cell transplant matching.
Older lab methods rely on PCR (a technique that copies and amplifies specific DNA segments) paired with probes that detect known HLA sequences. These approaches work well but have limitations. They can miss rare alleles entirely, and sometimes one allele copy gets amplified while the other is lost during processing, a problem called allele dropout.
Next-generation sequencing (NGS) has largely addressed these shortcomings. NGS reads entire HLA gene regions, including sections that older methods skip, and does so for multiple genes simultaneously. It achieves about 99% accuracy for identifying specific alleles and can detect novel variants that aren’t in existing databases. When researchers compared NGS results to older PCR-based methods, the two agreed only about 90-93% of the time, with most discrepancies reflecting limitations of the older technique rather than errors in NGS. For transplant programs that need the highest confidence in a match, NGS has become the preferred method.
What Your Results Look Like
HLA typing results are reported as a series of gene names followed by numbers. A low-resolution result might read “HLA-A*02,” meaning you carry a variant in the A02 family. A high-resolution result adds more digits, such as “HLA-A*02:01,” specifying the exact protein variant. For transplant matching, both copies of each gene (one from each parent) are reported, so a full high-resolution profile lists two results per locus across multiple genes.
Your HLA type is fixed at birth and never changes, so testing only needs to be done once. If you’ve been typed for one purpose, such as joining a bone marrow donor registry, that result remains valid for any future clinical need, though a lab may re-confirm results at higher resolution if a transplant match is identified.