HLA typing is a laboratory test that identifies the specific set of proteins on your cells that your immune system uses to distinguish your own tissue from foreign invaders. These proteins, called human leukocyte antigens, exist in millions of possible combinations, making each person’s HLA profile nearly unique. The test is most commonly used to match organ and bone marrow donors with recipients, but it also plays a role in diagnosing certain diseases and screening for dangerous drug reactions.
What HLA Proteins Actually Do
Every nucleated cell in your body displays HLA proteins on its surface. Think of them as molecular ID badges. Your immune system’s T-cells constantly scan these badges to determine whether a cell belongs to you or is a threat. When a cell is infected by a virus or becomes cancerous, HLA proteins grab fragments of the abnormal proteins inside the cell and display them on the surface, essentially flagging the cell for destruction.
The genes that code for HLA proteins are the most variable in the entire human genome. The region near the protein’s binding groove, where it grabs and presents those fragments, is especially diverse. This variation changes how effectively different people’s immune systems recognize different pathogens, which is one reason some people are more susceptible to certain infections or autoimmune diseases than others.
You inherit two sets of HLA genes, one from each parent, for a total of 12 key HLA markers. These markers fall into two classes: Class I proteins (HLA-A, HLA-B, and HLA-C) appear on nearly all cells, while Class II proteins (including HLA-DR, HLA-DQ, and HLA-DP) appear mainly on immune cells. Together, they create a profile that is almost as individual as a fingerprint.
Why HLA Typing Matters for Transplants
The most high-stakes use of HLA typing is matching donors and recipients for bone marrow or stem cell transplants. Because your immune system uses HLA markers to identify “self,” transplanted cells with different markers can trigger a severe immune attack in either direction: the recipient’s body may reject the donor cells, or the donor cells may attack the recipient’s tissues.
Matching requirements vary by donor type. For an unrelated adult donor, doctors look for a full match at eight key markers (HLA-A, HLA-B, HLA-C, and HLA-DRB1), scored as 8 out of 8. A matched sibling donor is evaluated at six markers (HLA-A, HLA-B, and HLA-DRB1), scored as 6 out of 6. Cord blood transplants have more flexibility, requiring a minimum match of 4 out of 6. Advances in transplant medicine have also made haploidentical (half-match) transplants viable for some patients, typically from a parent or sibling who shares at least half of the recipient’s markers.
Each sibling who shares the same biological parents has a 25% chance of being a full HLA match. When no family match exists, doctors turn to registries. The U.S. registry maintained through the NMDP contains more than 9.4 million potential donors and roughly 247,900 available cord blood units. Finding a match is harder for patients from underrepresented backgrounds because HLA patterns vary by ancestry. About 54% of U.S. registry volunteers identify as white non-Hispanic, while Black or African American donors make up 8%, Hispanic or Latino donors 13%, and Asian donors 10%.
HLA Links to Autoimmune Disease
Certain HLA variants dramatically increase the risk of specific autoimmune conditions. HLA typing can help confirm or rule out a diagnosis when symptoms are ambiguous.
- Ankylosing spondylitis: The HLA-B27 marker appears in 96% of patients with this inflammatory spinal condition. A negative test makes the diagnosis much less likely.
- Celiac disease: Virtually all celiac patients carry either HLA-DQ2 or HLA-DQ8. About 30% to 35% of the general population carries one of these markers, but only 2% to 5% of carriers actually develop the disease. The real power of this test is exclusion: if you lack both markers, your chance of having celiac disease is essentially zero.
- Type 1 diabetes: Specific HLA-DQ variants are strongly linked to susceptibility, and HLA typing is sometimes used in research settings to assess risk in relatives of people with the condition.
- Rheumatoid arthritis: Multiple HLA-DRB1 variants increase risk, and the specific variants differ across ethnic groups. Some HLA-DRB1 variants are actually protective.
- Multiple sclerosis: The HLA-DRB1*15:01 variant is the strongest single genetic risk factor identified for MS in people of European and Latin American descent.
Carrying a risk-associated HLA type does not mean you will develop the disease. These markers indicate genetic susceptibility, not destiny. Environmental triggers, other genes, and chance all play a role.
Drug Safety Screening
HLA typing is now required before prescribing certain medications that can cause life-threatening reactions in people with specific HLA variants. The most established example involves abacavir, an HIV medication. People who carry the HLA-B*57:01 marker face a significant risk of a severe hypersensitivity reaction, so guidelines strongly recommend testing before prescribing the drug. If you test positive, abacavir should not be prescribed, and the result is recorded as a drug allergy in your medical record.
This test only needs to be done once in your lifetime since your HLA type never changes. It is worth noting that the test is better at identifying who is at risk than at predicting who will definitely react. Roughly 33% to 50% of people who carry HLA-B*57:01 would likely tolerate abacavir without problems, but the potential severity of the reaction makes avoidance the standard approach. Similar HLA-based screening is used for other medications, including certain anti-seizure drugs.
How the Test Works
HLA typing requires only a blood draw or a cheek swab. The sample provides DNA, which the lab analyzes to identify your specific HLA alleles. No special preparation is needed, and results typically come back within days to a few weeks depending on the resolution required.
The technology behind HLA typing has evolved significantly. Older methods used probes or primers designed to detect known HLA sequences, but these approaches could not always distinguish between closely related variants. Since around 2016, next-generation sequencing has become the gold standard. This technique reads long stretches of DNA across HLA genes, including regions between the coding sections, which resolves ambiguities that older methods missed. It is especially critical for transplant matching, where even small differences between donor and recipient can affect outcomes.
Reading an HLA Result
HLA alleles follow a standardized naming system that packs a lot of information into a short code. Take HLA-DRB1*13:01:02 as an example. “HLA-DRB1” identifies the gene. The asterisk separates the gene name from the allele designation. The first number (13) indicates the broad type, which historically corresponded to the protein detected by older blood-based tests. The second number (01) identifies the specific subtype, meaning the protein sequence differs from other DRB1*13 variants. The third number (02) indicates a “silent” DNA change that does not alter the protein itself.
A fourth set of digits, when present, marks differences in non-coding DNA regions flanking the gene. Suffix letters carry additional meaning: “N” flags a null allele that produces no protein, while “L” indicates an allele with unusually low surface expression. For most clinical purposes, the first two sets of digits (the type and subtype) are what matter. High-resolution typing, which identifies the exact subtype, is the standard for transplant matching because two alleles that look identical at low resolution can differ at the protein level in ways that provoke an immune response.