Home HLA and Immune Genetics HLA-C Genetic Test: Tissue Type, Immune Matching, and Results

HLA-C Genetic Test: Tissue Type, Immune Matching, and Results

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Understand HLA-C genetic testing for tissue type, stem cell matching, donor-specific antibodies, KIR ligands, allele results, mismatches, limitations, and transplant follow-up.

An HLA-C genetic test identifies the two HLA-C alleles a person inherited and helps describe immune-system tissue type. HLA-C is a class I human leukocyte antigen found on most nucleated cells. Its main clinical use is donor–recipient matching for blood or marrow stem cell transplantation, where an HLA-C mismatch can increase graft-versus-host disease, graft failure, and mortality risk in some transplant settings. HLA-C results can also help evaluate donor-specific antibodies in solid-organ transplantation and support specialized research or immune-genetics questions involving natural killer cells. The report is not graded as simply normal or abnormal. Almost everyone has two valid HLA-C alleles, and the clinical meaning comes from comparing the result with another person’s HLA type, an antibody profile, or a specific disease association. High-resolution sequencing is often needed because closely related alleles can differ at medically important positions. A match does not guarantee transplant success, and a mismatch does not automatically rule out a donor; the transplant team weighs the full HLA profile, antibodies, donor factors, disease urgency, and planned graft-versus-host disease prevention.

  • HLA-C testing reports inherited tissue-type alleles, not a positive or negative disease result.
  • For unrelated stem cell transplantation, HLA-C is commonly included with HLA-A, HLA-B, and HLA-DRB1 in an 8/8 high-resolution match.
  • A full HLA-C allele match can reduce alloreactivity risk, but transplant outcomes also depend on other HLA loci and clinical factors.
  • No fasting is needed; testing usually uses blood, saliva, or a cheek swab.
  • High-resolution results use names such as HLA-C07:01 or HLA-C07:02, which are not interchangeable.
  • HLA-C genotype alone does not diagnose autoimmune disease, infection risk, infertility, or transplant rejection.

Table of Contents

What HLA-C Is and What the Test Measures

HLA-C is one of the classical HLA class I genes, alongside HLA-A and HLA-B. These genes are located close together on chromosome 6 and encode proteins that display short peptide fragments at the cell surface. Cytotoxic T cells inspect those peptide–HLA complexes for signs of infection or abnormal cellular activity. Natural killer, or NK, cells also recognize HLA-C through killer cell immunoglobulin-like receptors, known as KIRs.

HLA genes are highly polymorphic, meaning thousands of allele sequences exist across human populations. Each person normally inherits one HLA-C allele from each biological parent. The two alleles form that person’s HLA-C genotype. Examples include HLA-C07:01 and HLA-C15:02.

The test determines allele identity at a chosen level of resolution. Low-resolution typing may report an allele group such as HLA-C07. High-resolution typing usually reports at least two fields, such as HLA-C07:01. Additional fields can distinguish synonymous DNA changes or noncoding differences. For clinical matching, laboratories report the resolution required by the transplant program and current standards.

HLA-C proteins are expressed at lower levels on many cells than HLA-A or HLA-B, but lower expression does not make the locus unimportant. HLA-C mismatch can stimulate donor T cells, recipient T cells, NK cells, or antibodies depending on the transplant direction and context. Some HLA-C alleles also differ in surface-expression level, peptide repertoire, and KIR binding.

The result is a tissue-type description rather than a health score. There is no universal “best” HLA-C allele and no normal reference range. An allele common in one ancestry group may be uncommon in another without being harmful. Clinical interpretation requires a comparison or a defined question.

A standalone HLA-C test is less common than a multi-locus HLA typing test. Transplant laboratories usually type HLA-A, HLA-B, HLA-C, HLA-DRB1, and often HLA-DQB1 and HLA-DPB1 together. Targeted HLA-C testing may be used to confirm an ambiguity, type a relative, investigate an antibody specificity, or support KIR-focused analysis.

Why HLA-C Is Tested

The leading clinical reason is donor selection for allogeneic hematopoietic cell transplantation. In an allogeneic transplant, stem cells come from another person. The donor immune system must rebuild inside the recipient while avoiding excessive attack on recipient tissues. Close HLA matching lowers—but does not eliminate—the chance of graft-versus-host disease and other immune complications.

HLA-C testing may be ordered for:

  • a patient preparing for blood or marrow stem cell transplantation
  • biological siblings or other relatives being evaluated as donors
  • unrelated volunteer donors in a registry search
  • cord blood units under consideration for transplant
  • a solid-organ donor and recipient when detailed HLA typing is needed
  • evaluation of anti-HLA-C antibodies or donor-specific antibodies
  • confirmation of a previously reported HLA type
  • KIR–HLA studies in selected transplant or research programs
  • investigation of a specific disease-associated HLA-C allele when clinically justified

A transplant donor is never selected from HLA-C alone. The team compares the entire relevant HLA profile. It also considers donor age, health, sex and pregnancy history, cytomegalovirus status, blood group, stem-cell source, availability, timing, and the recipient’s antibodies.

For solid-organ transplantation, HLA-C historically received less emphasis than HLA-A, HLA-B, and HLA-DR. Modern antibody assays and high-resolution typing show that HLA-C antibodies can be clinically relevant, especially when they are donor-specific. The degree of importance depends on organ type, antibody strength and characteristics, crossmatch findings, and center protocol.

HLA-C testing can also appear in immune-genetics workups unrelated to transplantation. Certain HLA-C alleles have statistical associations with psoriasis, HIV control, inflammatory conditions, reproductive outcomes, or drug reactions. These associations usually have limited predictive value for an individual and are not equivalent to diagnosis. A broad consumer or wellness interpretation can overstate what the allele means.

How HLA-C Typing Is Performed

HLA-C typing usually requires a blood sample, cheek swab, or saliva sample. Fasting is unnecessary. The laboratory extracts DNA and amplifies HLA-C regions before identifying allele sequences.

Common methods include:

  • Sequence-specific oligonucleotide probes: detect patterns of known HLA variants and often provide low to intermediate resolution.
  • Sequence-specific primer PCR: uses allele-targeted primers and can provide rapid focused typing.
  • Sanger sequence-based typing: reads selected HLA-C exons but may leave phase ambiguities when variants could occur on either chromosome copy.
  • Next-generation sequencing: reads longer regions and can provide high-resolution, phased allele assignments.
  • Long-read sequencing: can span most or all of the gene and resolve difficult ambiguities, though it is not required for every case.

Turnaround time ranges from hours for urgent family typing to several days or weeks for comprehensive registry or confirmatory testing. Emergency transplant searches may use rapid preliminary typing followed by verification at higher resolution.

HLA nomenclature is updated as new alleles are discovered. A laboratory may revise an allele name when the international HLA database changes, even though the person’s DNA has not changed. Reports should identify the database version and method when precise comparisons matter.

Sample source requires attention after an allogeneic stem cell transplant. Blood may contain donor-derived white cells and therefore report the donor’s HLA type. To determine the recipient’s original genotype, the laboratory may need a pre-transplant sample, archived DNA, buccal cells with contamination controls, hair roots, or cultured fibroblasts. After solid-organ transplantation, blood generally remains recipient-derived.

HLA typing and HLA antibody testing are different. Typing identifies inherited alleles. Antibody testing examines whether a recipient has antibodies that recognize certain HLA antigens. A complete compatibility assessment may require both, plus a crossmatch test using donor cells or a virtual comparison.

Identity and chain-of-custody checks are especially important for donor workups. A preliminary registry sample may be enough to rank candidates, but the selected donor normally undergoes confirmatory typing from a new independently collected specimen. This reduces the chance of sample mix-up, transcription error, or outdated low-resolution data. If the confirmatory result differs, the transplant team recalculates the match before collection begins.

Family typing can also reveal inheritance patterns that help resolve an ambiguous allele pair. A child receives one HLA haplotype from each parent, so parental or sibling results may show which HLA-C allele travels with neighboring HLA-A and HLA-B alleles. Laboratories use this segregation information as supporting evidence, not as a substitute for adequate sequencing.

Reading an HLA-C Result

A typical high-resolution result lists two alleles, for example:

  • HLA-C*04:01
  • HLA-C*07:02

The first field identifies an allele family; the second generally distinguishes protein sequence. Additional fields may appear, such as HLA-C*07:02:01:03. Suffixes can indicate expression characteristics, including null alleles that are not expressed or are expressed abnormally.

If both inherited copies are the same, the person is homozygous at HLA-C. The report may list the allele twice or note homozygosity. Homozygosity is not automatically harmful. In matching, it can create directional differences: a donor and recipient may appear matched in one direction but still have an alloreactive difference in the other depending on the exact alleles and transplant design.

Results may be reported as:

  • Unambiguous: one allele pair fits the sequence data.
  • Ambiguous: more than one allele pair could explain the observed sequence.
  • Probable: a statistical or population-based assignment was made pending confirmation.
  • Not tested or unresolved: the assay did not provide adequate information.
  • Novel allele: the sequence differs from known database entries and requires verification.

An allele mismatch means the donor and recipient have different high-resolution alleles at HLA-C. An antigen mismatch is broader and may involve different first-field groups. Not all mismatches produce equal immune effects. Amino-acid position, expression level, peptide-binding differences, direction of mismatch, and the recipient’s antibodies can modify risk.

A report may also identify HLA-C1 or HLA-C2 KIR ligand groups. Most HLA-C molecules carry one of two motifs recognized by inhibitory KIR receptors. C1 and C2 are not “good” and “bad” categories. They describe receptor-ligand biology that may be considered in selected NK-cell or transplant analyses.

Results from two laboratories should be compared at the same resolution. HLA-C07 is not enough to prove a match with HLA-C07:01, because the other person could carry HLA-C*07:02 or another allele in the same group. Verification typing reduces this error before donation.

HLA-C in Stem Cell Transplant Matching

For many unrelated adult donor transplants, a conventional high-resolution match evaluates HLA-A, HLA-B, HLA-C, and HLA-DRB1. Two alleles at each of four loci create an 8/8 match. Some programs describe a 10/10 match by adding HLA-DQB1, while others add HLA-DPB1 and further loci for extended assessment.

An 8/8 donor has matching high-resolution alleles at both copies of HLA-A, HLA-B, HLA-C, and HLA-DRB1. A 7/8 donor has one mismatch among those eight allele comparisons. Historically, a single HLA-C mismatch was associated with higher acute graft-versus-host disease and mortality in some conventional unrelated-donor settings. That is why HLA-C remains a core locus in donor selection.

Contemporary transplant methods have changed the weight of mismatch. Post-transplant cyclophosphamide and other graft-versus-host disease prophylaxis strategies allow successful haploidentical and mismatched unrelated donor transplantation. Donor age, speed of availability, disease urgency, donor-specific antibodies, and the planned prophylaxis can sometimes outweigh the pursuit of a perfect match.

NMDP guidance still prioritizes an 8/8 HLA-A, -B, -C, and -DRB1 match when one is likely to be available without harmful delay. When it is not, the team evaluates alternative donors rather than searching indefinitely. Certain HLA-C mismatches, such as HLA-C03:03 versus HLA-C03:04, may be considered more permissible in specific settings because the proteins differ only slightly, but this is a transplant-specialist judgment.

The direction of mismatch can matter. In graft-versus-host direction, donor immune cells recognize a recipient HLA difference. In host-versus-graft direction, recipient immunity recognizes donor cells. A bidirectional mismatch can affect both. Conditioning intensity and graft source modify these effects.

HLA-C expression adds another layer. Some alleles are expressed more strongly at the cell surface. A mismatch involving a highly expressed recipient HLA-C allele may present more target for donor T cells than a lower-expression mismatch. Expression is not routinely used as a simple standalone cutoff, but it helps explain why mismatches differ biologically.

No HLA match guarantees success. Even fully matched donor and recipient pairs differ at minor histocompatibility antigens and many non-HLA genes. Infection, disease status, age, organ function, conditioning, graft cell dose, and post-transplant immune management remain major determinants.

HLA-C Antibodies and Solid-Organ Transplantation

An HLA-C mismatch can become clinically important when a recipient has antibodies directed against the donor’s HLA-C molecule. These donor-specific antibodies, or DSAs, may arise after pregnancy, transfusion, or prior transplantation. Some people develop them without a clearly documented sensitizing event.

Single-antigen bead assays can detect anti-HLA-C antibody reactivity. The laboratory reports patterns and often a semiquantitative signal called mean fluorescence intensity. MFI is not a universal concentration or a stand-alone measure of pathogenicity. Results vary with assay conditions, interference, dilution, and laboratory thresholds.

To decide whether an antibody is donor-specific, the laboratory compares the recipient’s antibody profile with the donor’s HLA type. High-resolution donor typing may clarify whether an apparent HLA-C antibody truly recognizes that donor allele. A donor-specific antibody test integrates these data.

HLA-C DSAs have been associated with antibody-mediated rejection and graft injury in kidney and other solid-organ transplants, but risk is context dependent. The team considers antibody persistence, complement-binding features, crossmatch results, previous sensitization, organ type, and the total antibody profile. A weak isolated bead signal does not automatically prohibit transplantation.

HLA-C is expressed on donor cells and vascular endothelium, though often at lower levels than HLA-A and HLA-B. Lower expression may partly explain why some HLA-C antibodies behave differently, but it does not make them irrelevant. A clinically significant positive crossmatch or strong DSA requires transplant-center interpretation.

After transplant, new or rising HLA-C DSA can prompt closer monitoring, kidney or organ-function tests, biopsy, or treatment assessment. The genotype itself has not changed; the immune response to the donor has.

HLA-C, KIR, and Disease Associations

HLA-C has a distinctive role as a ligand for KIR receptors on NK cells. HLA-C1 molecules generally interact with KIR2DL2 and KIR2DL3, while HLA-C2 molecules interact with KIR2DL1. Activating KIRs can recognize related motifs with more complex and variable effects. The combination of a person’s KIR genes and HLA ligands helps tune NK-cell education and response.

KIR–HLA analysis is studied in stem cell transplantation, pregnancy, infection, cancer, and autoimmune disease. Some transplant programs consider donor KIR genotype or KIR-ligand mismatch in selected diseases, especially acute myeloid leukemia, but evidence and protocols vary. It is not a substitute for core HLA matching or DSA avoidance. A separate KIR genetic test is required because HLA-C typing does not reveal which KIR genes a person carries.

HLA-C06:02 has a strong population association with psoriasis, particularly earlier-onset plaque psoriasis. Even so, many carriers never develop psoriasis, and many patients do not carry the allele. Routine HLA-C06:02 testing is not required to diagnose psoriasis. Research has explored whether it predicts response to specific biologic therapies, but it is not a universal prescribing test.

Certain HLA-C variants have also been associated with HIV viral control, Behçet disease, inflammatory bowel disease, and reproductive outcomes. These are probabilistic associations shaped by linked HLA variants and population structure. A result should not be converted into a personalized disease forecast without a validated clinical model.

HLA-C can be inherited in a haplotype with particular HLA-B or HLA-DR alleles. An observed association may therefore reflect linkage disequilibrium rather than a direct effect of HLA-C. This is one reason disease-association reports require cautious interpretation and ancestry-matched evidence.

Limitations and Next Steps

HLA-C testing is technically challenging because the locus is highly variable and closely related to other HLA class I genes. Allele dropout, phase ambiguity, incomplete gene coverage, contamination, and outdated reference databases can affect results. High-quality transplant laboratories use controls, repeat testing, and independent verification before donation.

The test cannot by itself determine:

  • whether a donor will be accepted
  • whether graft-versus-host disease or rejection will occur
  • whether an HLA-C antibody will injure a graft
  • whether a person will develop an HLA-associated disease
  • whether an NK-cell response will be beneficial
  • the best conditioning or immunosuppression regimen

When a potential donor appears mismatched, the next step may be extended high-resolution typing, antibody review, a virtual or physical crossmatch, or comparison with other donors. The transplant team may also assess HLA-DPB1 permissiveness, donor age, and alternative graft sources.

When the test was ordered for a disease association, the clinician should ask whether the result changes diagnosis or management. If not, additional testing may add anxiety without clinical benefit. Genetic counseling can help when patients interpret inherited HLA findings as proof of disease or family relationships.

A new HLA-C result should be compared with prior reports using exact allele names and database versions. Apparent discrepancies often reflect higher resolution rather than an actual conflict. For example, an older HLA-C07 result can be compatible with a newer HLA-C07:02 result.

The most useful interpretation is question-specific: match the donor, identify a DSA target, resolve an ambiguous HLA type, or analyze a validated KIR–ligand relationship. Without that context, an HLA-C allele list is descriptive but not medically actionable.

An HLA-C report may also classify the allele as a KIR ligand. C1 and C2 groups describe an amino acid feature recognized by selected natural killer cell receptors. This information is useful in research and some donor-selection models, but it is not the same as conventional HLA-C allele matching.

For transplant antibody assessment, allele-level typing can clarify whether an anti-HLA-C bead pattern is donor-specific. HLA-C is expressed at lower surface levels than HLA-A or HLA-B in many cells, yet anti-C antibodies can still be clinically relevant. MFI alone should not be used to dismiss or confirm risk.

Patients comparing donor reports should verify whether the score includes both HLA-C alleles and whether typing was confirmatory. A “close family match” based on a screening swab is not final until the transplant laboratory confirms identity and high-resolution results. Donor age, availability, health, antibodies, disease urgency, and prophylaxis remain part of selection.

References

Disclaimer

This article is educational and does not replace interpretation by a histocompatibility laboratory or transplant team. HLA-C results must be considered with the full HLA profile, antibody testing, crossmatch findings, donor characteristics, and the transplant protocol. Do not make donor or treatment decisions from a standalone HLA-C allele report.

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