An HLA blood test identifies the specific set of proteins on the surface of your cells that your immune system uses to distinguish “self” from “foreign.” These proteins, encoded by human leukocyte antigen genes on chromosome 6, are among the most variable in the entire human genome, meaning your particular combination is nearly unique to you. The test matters because that combination determines whether a transplanted organ or donated stem cells will be accepted or attacked, whether you carry genetic risk for certain autoimmune diseases, and whether specific medications could trigger a dangerous reaction. Few single lab tests reach into so many corners of medicine.
What HLA Proteins Actually Do
Your immune system needs a way to inspect cells for signs of trouble. HLA proteins are the display cases that make this possible. They sit on the outer surface of nearly every cell in your body, holding small fragments of proteins from inside the cell and presenting them to passing immune cells. If those fragments look normal, the immune cell moves on. If a fragment looks foreign, say, a piece of a virus that has infected the cell, the immune system mounts an attack.
The genes that code for these proteins are clustered in a stretch of DNA on chromosome 6 spanning roughly five million base pairs, making it one of the most gene-dense and variable regions in the human genome.1Frontiers in Bioinformatics. DNA structural features and variability of complete MHC locus sequences The region is split into three classes. Class I genes (HLA-A, HLA-B, and HLA-C) produce proteins found on almost all nucleated cells. Class II genes (HLA-DR, HLA-DQ, and HLA-DP) produce proteins mainly found on immune cells like macrophages and B cells. Class III genes encode other immune components, including certain complement proteins, but are less commonly the target of clinical HLA testing.
What makes HLA remarkable is its diversity. It is the most polymorphic genetic system known in humans, with thousands of documented variants across the different genes.2PubMed Central. The HLA system: genetics, immunology, clinical testing, and clinical implications Two unrelated people are unlikely to share an identical HLA profile. Even siblings have only about a one-in-four chance of inheriting the same set of HLA genes from both parents. This extreme variability is the whole reason the test exists: because your HLA type is so specific, matching it to a donor or screening it for disease-linked variants yields clinically useful information.
How the Test Is Done
From your perspective, an HLA test is a straightforward blood draw. A few tubes of blood are sent to a specialized laboratory, and the results typically come back within days to a couple of weeks, depending on the resolution needed and the lab’s workload. What happens behind the scenes, though, has changed dramatically over the decades.
Early HLA typing relied on serological methods: mixing a patient’s cells with known antibodies and observing whether the cells were killed, which indicated the presence of a specific HLA antigen. These techniques were groundbreaking when they launched in the 1960s but were limited in their ability to distinguish closely related variants. Modern labs use DNA-based methods, including next-generation sequencing, which read the actual genetic code of HLA genes. These advances have sharply improved accuracy and reduced the ambiguities that once complicated transplant matching.3PubMed Central. A walk through the development of human leukocyte antigen typing: from serologic techniques to next-generation sequencing For most transplant-related testing, labs now perform high-resolution typing that identifies exact allele variants rather than broad antigen groups.
A cheek swab can also provide DNA for HLA typing, and some donor registries use swab kits for initial enrollment. But blood samples remain the standard for clinical decisions because they allow additional tests, such as crossmatching a patient’s serum against a potential donor’s cells to check for pre-existing antibodies.
Stem Cell and Bone Marrow Transplants
Matching HLA types between a patient and donor is the single most important factor in the success of a bone marrow or stem cell transplant. When HLA proteins on the donated cells look foreign to the patient’s immune system, the patient may reject the graft. And in a twist unique to this type of transplant, the donated immune cells can also attack the patient, a condition called graft-versus-host disease (GVHD). A retrospective analysis of over 10,000 transplants confirmed that donor HLA matching was a key risk factor for both acute GVHD and overall survival.4PubMed Central. Which factors influence the development of GVHD in HLA-matched or mismatched transplants? – Section: Patient-donor matching
The gold standard is a 10-out-of-10 match, meaning both copies of HLA-A, HLA-B, HLA-C, HLA-DRB1, and HLA-DQB1 are identical between donor and patient at high resolution.5PubMed Central. How to select the best available related or unrelated donor of hematopoietic stem cells? Even a single mismatch at one of these loci raises the risk of complications. When multiple mismatches are present, the hazard climbs steeply: one study found that patients with multiple HLA mismatches had roughly double the risk of death and about two-and-a-half times the risk of severe acute GVHD compared with well-matched pairs.6PubMed. HLA Association with hematopoietic stem cell transplantation outcome: the number of mismatches at HLA-A, -B, -C, -DRB1, or -DQB1 is strongly associated with overall survival
This is why patients in need of a transplant who lack a matched sibling are directed to large volunteer donor registries, where millions of people have been HLA-typed. The search can still be difficult, and for some patients no adequate match exists, a problem that connects directly to disparities in registry composition discussed further below.
Solid Organ Transplants
HLA matching also matters when transplanting kidneys, hearts, lungs, and livers, although its weight relative to other factors varies by organ. In kidney transplantation the relationship is especially well documented. HLA mismatches between donor and recipient reduce graft survival and can lengthen the time a patient spends on dialysis waiting for a better-matched organ.7PubMed Central. Kidney Transplantation: The Challenge of Human Leukocyte Antigen and Its Therapeutic Strategies
A major concern is the formation of donor-specific antibodies (DSA), which are antibodies your immune system makes against the donor’s HLA proteins. These can develop after exposure to foreign HLA through blood transfusions, previous transplants, or pregnancy. When present before transplant, DSA can cause rapid rejection. A large Korean registry study found that patients who carried pre-transplant DSA developed antibody-mediated rejection at roughly four times the rate of those without DSA.8Frontiers in Immunology. Pre-transplant crossmatch-negative donor-specific anti-HLA antibody predicts acute antibody-mediated rejection but not long-term outcomes in kidney transplantation DSA can also develop after transplant. Research has shown that the degree of mismatch at specific HLA subunits, particularly at DQ, is linearly associated with the odds of forming new DSA, rejection, and graft failure, with no safe threshold below which the risk disappears entirely.9PubMed Central. Eplet Mismatch Load and De Novo Occurrence of Donor-Specific Anti-HLA Antibodies, Rejection, and Graft Failure after Kidney Transplantation
This is why HLA testing before organ transplantation includes not just typing your HLA alleles but also screening your blood for pre-formed antibodies. The combination lets transplant teams predict which donors are safe and which are likely to trigger an immune crisis.
Diagnosing Autoimmune Conditions
Certain HLA variants are strongly linked to specific autoimmune diseases, and testing for them has become a routine part of diagnosis in several conditions. The best-known example is the connection between HLA-B27 and ankylosing spondylitis, a form of inflammatory arthritis that primarily affects the spine. About 60 to 90 percent of patients with axial spondyloarthritis worldwide carry the HLA-B27 allele, and testing for it is now considered a cornerstone of diagnosis and classification.10RMD Open. Fifty years after the discovery of the association of HLA B27 with ankylosing spondylitis – Section: Part II clinical implications of determining HLA B27 in patients under suspicion of axSpA A positive result does not diagnose the disease on its own, because HLA-B27 is common in the general population too. Instead, the test shifts the probability: in a patient with suggestive back pain and imaging findings, a positive HLA-B27 result makes the diagnosis considerably more likely, while a negative result makes it less so.11PubMed. Diagnostic value of HLA-B27 testing ankylosing spondylitis and Reiter’s syndrome
Celiac disease offers a different use case. Almost all people with celiac disease carry HLA-DQ2 or HLA-DQ8, so testing for these alleles is a powerful way to rule the condition out rather than in. If you test negative for both, the chance you have celiac disease is extremely low. One clinical study found a negative predictive value of 98 percent among patients tested for these alleles.12PubMed Central. Clinical utility of celiac disease associated HLA testing Meta-analyses have confirmed that DQ2/DQ8 typing is effective at excluding celiac disease from the differential diagnosis when symptoms are ambiguous.13PubMed Central. Meta-Analysis and Systematic Review of HLA DQ2/DQ8 in Adults with Celiac Disease Because so many people carry these alleles without ever developing celiac disease, however, a positive result just means the door is open, not that you have walked through it.
Type 1 diabetes follows a similar pattern. The strongest genetic risk factors are specific HLA-DR and HLA-DQ haplotypes, commonly abbreviated as DR3 and DR4.14PubMed Central. Genetics of the HLA region in the prediction of type 1 diabetes In rheumatoid arthritis, HLA-DR4 and HLA-DR1 alleles that share a particular amino acid sequence (sometimes called the shared epitope) have been associated with disease susceptibility.15Clinical Biochemistry. HLA molecules in autoimmune diseases In research settings, HLA typing is used to stratify risk and identify who might benefit from monitoring or early intervention, though it is not yet standard screening for the general population in either condition.
Preventing Dangerous Drug Reactions
One of the most directly actionable uses of HLA testing is in pharmacogenomics: screening for HLA variants that predict severe drug reactions before you ever take the medication. Two examples stand out.
Abacavir is an antiretroviral drug used in HIV treatment. Roughly five to eight percent of people who take it develop a serious hypersensitivity reaction, and carrying the HLA-B*5701 allele is the major risk factor. A landmark trial showed that screening patients for HLA-B*5701 before prescribing abacavir completely eliminated confirmed hypersensitivity reactions in the screened group, compared with a roughly three-percent incidence in the unscreened group.16PubMed. HLA-B*5701 screening for hypersensitivity to abacavir The negative predictive value was 100 percent, meaning every person who tested negative for the allele was safe to take the drug.17PubMed Central. HLA-B*5701 testing to predict abacavir hypersensitivity Pre-prescription HLA-B*5701 testing is now standard of care worldwide for anyone being considered for abacavir.
Carbamazepine, an antiepileptic drug also used for nerve pain and mood disorders, poses a different risk. In people of East and Southeast Asian descent, the HLA-B*1502 allele is strongly linked to Stevens-Johnson syndrome and toxic epidermal necrolysis, both of which are life-threatening skin reactions. The association is dramatic: meta-analysis found an odds ratio above 80 for bullous skin lesions in Asian carriers of HLA-B*1502.18Pharmacogenetics and Genomics. HLA alleles and hypersensitivity to carbamazepine: an updated systematic review with meta-analysis The U.S. FDA now recommends that all patients of Asian ancestry be genotyped for HLA-B*1502 before starting carbamazepine.19PubMed Central. Carbamazepine, HLA-B*1502 and risk of Stevens-Johnson syndrome and toxic epidermal necrolysis: US FDA recommendations A separate allele, HLA-A*3101, has been identified as a broader risk marker for carbamazepine-related skin reactions across populations.
Platelet Transfusion Refractoriness
Most people associate blood transfusions with red blood cells, but patients undergoing chemotherapy or major surgery often need platelet transfusions to prevent bleeding. Some patients develop antibodies against the HLA class I proteins on donor platelets, which causes the transfused platelets to be cleared from the bloodstream almost immediately. This condition, called platelet refractoriness, is a serious clinical problem because it leaves the patient without clotting protection and at high risk of major bleeding.20PubMed Central. Anti-HLA Class I alloantibodies in platelet transfusion refractoriness: From mechanisms and determinants to therapeutic prospects
Exposure to foreign HLA through pregnancy, prior transfusions, or organ transplants is the primary way people develop these antibodies.21American Journal of Clinical Pathology. HLA-Mediated Platelet Refractoriness: An ACLPS Critical Review – Section: Management When refractoriness is confirmed, the solution is to find platelets that are HLA-compatible with the patient. Traditionally this meant locating a donor whose HLA-A and HLA-B antigens closely matched the patient’s, which required maintaining a registry of HLA-typed platelet donors. Newer approaches use the patient’s specific antibody profile to select donors whose platelets simply lack the antigens the patient has antibodies against, a strategy that substantially expands the pool of usable donors compared to strict HLA matching alone.22Hematology Am Soc Hematol Educ Program. Platelet transfusion refractoriness: how do I diagnose and manage? – Section: Identifying compatible platelet units
HLA, Cancer, and Immunotherapy
Cancer immunotherapy, especially immune checkpoint inhibitors, has transformed oncology in the past decade. These drugs work by releasing the brakes on your immune system so it can recognize and destroy tumor cells. But immune recognition of a tumor depends on HLA molecules displaying abnormal protein fragments (neoantigens) from cancer cells. If a tumor loses its ability to display those fragments, it becomes invisible to the immune system.
Research has shown that many tumors accomplish exactly this escape by losing one copy of their HLA genes, a phenomenon called loss of heterozygosity (LOH). In lung cancer, one study found that up to 92 percent of predicted neoantigens in a single tumor were bound to the lost HLA type, meaning the immune system had no way to detect them.23Cell. Loss of Heterozygosity in Human Leukocyte Antigen Class I and Immune Escape in Lung Cancer – Section: Discussion Other tumors disable HLA presentation through mutations in a gene called B2M, which is necessary for HLA class I molecules to reach the cell surface. Knocking out this gene in mouse lung tumors was enough to confer resistance to PD-1 checkpoint blockade, directly proving the link between HLA loss and immunotherapy failure.24PubMed Central. Impaired HLA Class I Antigen Processing and Presentation as a Mechanism of Acquired Resistance to Immune Checkpoint Inhibitors in Lung Cancer
This line of research is pushing oncologists toward integrating HLA analysis into tumor profiling. If a patient’s tumor has already lost its HLA expression, checkpoint inhibitors alone may not work, and alternative approaches like engineered T-cell therapies targeting different pathways may be needed.
HLA and Infectious Disease
Because HLA proteins determine which pathogen fragments your immune cells see, different HLA types lead to different strengths and weaknesses against specific infections. The most striking example involves HIV. A small number of HIV-positive people, known as elite controllers, maintain undetectable viral loads without antiretroviral therapy. The HLA-B*57 allele is heavily overrepresented in this group.25PubMed Central. HLA-B*57 elite suppressor and chronic progressor HIV-1 isolates replicate vigorously and cause CD4+ T cell depletion in humanized BLT mice Research using humanized mice has shown that the viruses carried by these elite controllers are fully capable of causing disease, ruling out the idea that they are simply infected with weaker strains. The control appears to come from the host’s immune response itself rather than defective virus.
The mechanism seems to involve a two-pronged attack. HLA-B57 molecules present fragments of HIV’s internal Gag protein to immune cells with unusual effectiveness. When the virus mutates to escape this recognition, the mutations reduce its ability to replicate. Meanwhile, the immune cells continue to respond to the mutated versions, keeping the pressure on.26PubMed Central. HLA-B57/B*5801 human immunodeficiency virus type 1 elite controllers select for rare gag variants associated with reduced viral replication capacity and strong cytotoxic T-lymphocyte recognition Natural killer cells add a further layer: certain combinations of killer cell receptor alleles and HLA-B*57 correlate with delayed progression to AIDS.27Frontiers in Cellular and Infection Microbiology. Combined Effects of HLA-B*57/5801 Elite Suppressor CD8+ T Cells and NK Cells on HIV-1 Replication
The flip side is that HLA alleles protective against one threat can increase vulnerability to another. HLA-B27, for instance, is associated with strong immune responses to certain infections but also predisposes carriers to autoimmune diseases like ankylosing spondylitis. HLA-B57 follows a similar double-edged pattern.28PubMed Central. Human Leukocyte Antigen (HLA) and Immune Regulation: How Do Classical and Non-Classical HLA Alleles Modulate Immune Response to Human Immunodeficiency Virus and Hepatitis C Virus Infections?
Why HLA Diversity Varies Across Populations
If you have ever wondered why there are so many HLA variants in the first place, the answer traces back to millions of years of host-pathogen arms races. Populations exposed to a wider variety of infectious agents tend to have greater HLA diversity, particularly at the HLA-B gene, which appears to be under the strongest selective pressure from pathogens.29PubMed. Pathogen-driven selection and worldwide HLA class I diversity The diversity of HLA binding preferences has been shaped by the sequence diversity of the very pathogens HLA molecules present to the immune system, creating a co-evolutionary feedback loop.30PubMed Central. Mapping the landscape of host-pathogen coevolution: HLA class I binding and its relationship with evolutionary conservation in human and viral proteins
This evolutionary history also explains a pressing clinical problem: donor registry disparities. Because HLA diversity is not evenly distributed across populations, some groups have a much harder time finding matched donors. Modeling of the U.S. stem cell registry estimates that White European patients have roughly a 74 percent chance of finding an 8-out-of-8 matched unrelated donor, while African American patients have only about a 25 percent chance.31Transplantation and Cellular Therapy. Unrelated Stem Cell Donor HLA Match Likelihood in the US Registry Incorporating HLA-DPB1 Permissive Mismatching The gap reflects both lower representation of minority donors in the registry and the higher underlying HLA diversity in populations of African descent, which means more registered donors are needed to cover the wider range of possible HLA combinations.
South African data illustrate how targeted recruitment can narrow this gap. Modeling showed that as the registry of Black African donors grew larger, match probabilities climbed steeply, eventually approaching those of White donors at large registry sizes.32Blood Global Hematology. HLA haplotype frequency analysis reveals large patient benefits from stem cell donor recruitment in Black South African population Similar analyses in India have highlighted significant regional disparities in match likelihood driven by the country’s high genetic diversity and endogamy patterns, prompting calls for geographically targeted donor recruitment.33PubMed. HLA match probability in Indian regional population using umbilical cord blood units: Implications for optimal registry size and targeted donor recruitment
How HLA Was Discovered
The story of HLA begins not with humans but with mice. In the early twentieth century, researchers noticed that tumors transplanted between different mice were sometimes rejected. CC Little, working with inbred mouse strains, showed that genetic differences between the strains controlled rejection. Peter Gorer then identified a genetic system in mice, called H-2, that governed this process, laying the groundwork for finding the human equivalent.34PubMed Central. A historical perspective on HLA
The human system came into view in 1958 with the discovery of the first histocompatibility antigen, initially called “MAC.” Over the following decade, laboratories worldwide identified virtually all of the common HLA-A and HLA-B antigens, using antibodies found in the blood of people who had been transfused or had multiple pregnancies.35PubMed. Origins of the first HLA specificities The international histocompatibility workshops that standardized naming conventions during this period remain active today and are the reason HLA nomenclature, while complex, is at least consistent worldwide. Jean Dausset, who identified that first antigen, shared the 1980 Nobel Prize in Physiology or Medicine for the work, alongside Baruj Benacerraf and George Snell, whose contributions in mice had paved the way.