Killer Cell Immunoglobulin-Like Receptor (KIR) Test: Immune Genetics and Results

A killer cell immunoglobulin-like receptor, or KIR, test identifies inherited genes and sometimes alleles that regulate natural killer cells. Natural killer cells help control infected, stressed, and malignant cells. Their KIR receptors send inhibitory or activating signals after interacting with HLA class I molecules, especially HLA-C and selected HLA-B or HLA-A groups. A KIR result may be used in research, selected blood stem cell donor evaluations, reproductive immunology investigations, and studies of infection, cancer, or autoimmune disease. It is not a general measure of immune strength and does not diagnose infertility, recurrent miscarriage, cancer, or an immune disorder by itself. Reports may list each KIR gene as present or absent, classify the genotype as AA or B/x, calculate KIR B-content, or provide high-resolution alleles. Interpretation requires the relevant HLA ligands, the clinical setting, and the testing method. A blood or cheek sample is sufficient, no fasting is needed, and the inherited result does not change over time.

• KIR testing usually reports gene presence or absence; some newer tests also identify copy number and specific alleles.
• Inhibitory KIR help natural killer cells tolerate healthy self-cells, while activating KIR can support responses to altered cells.
• A KIR genotype has meaning only with its HLA ligands and clinical context; it is not simply favorable or unfavorable.
• KIR-guided donor selection remains specialized and is not a universal requirement for blood stem cell transplantation.
• No fasting, medication pause, or special timing is needed for blood, saliva, or cheek-swab collection.

Table of Contents

• How KIR genes and receptors work
• What a KIR test reports
• Sample collection and laboratory methods
• Interpreting KIR with HLA ligands
• Use in stem cell transplantation
• Pregnancy, disease associations, and research uses
• Limitations and common mistakes
• Next steps after a KIR result

How KIR genes and receptors work

KIR genes sit in a cluster on chromosome 19. They encode receptors found mainly on natural killer cells and on some T cells. Natural killer cells can destroy abnormal cells without first recognizing one unique antigen in the way conventional T cells do. Instead, they combine signals from many activating and inhibitory receptors.

KIR names describe structure and signaling. “2D” or “3D” indicates two or three immunoglobulin-like extracellular domains. “L” means a long cytoplasmic tail that usually delivers an inhibitory signal. “S” means a short tail that usually associates with an activating signaling protein. For example, KIR2DL1 is commonly inhibitory, while KIR2DS1 is activating.

Several framework genes are found in nearly everyone, including KIR3DL3, KIR3DP1, KIR2DL4, and KIR3DL2. Other genes vary in presence, copy number, and allele. This variation creates many genotypes.

KIR receptors often recognize groups of HLA class I molecules:

• KIR2DL1 commonly recognizes HLA-C molecules in the C2 group.
• KIR2DL2 and KIR2DL3 commonly recognize HLA-C molecules in the C1 group.
• KIR3DL1 recognizes HLA-B and some HLA-A molecules carrying the Bw4 epitope.
• KIR3DL2 can interact with selected HLA-A molecules and other ligands.

The interaction helps “educate” or license natural killer cells. An inhibitory KIR that encounters its self-HLA ligand during development can make the cell more responsive when that ligand later disappears from a stressed or malignant target. This is part of the “missing self” concept: a target cell that loses normal HLA class I expression may remove an inhibitory brake.

Activating KIR biology is more complex. Some activating receptors bind HLA weakly or only under particular peptide and cellular conditions. The presence of an activating gene does not mean the receptor is expressed on every natural killer cell or that the immune system is globally more active.

What a KIR test reports

KIR testing can range from a basic gene-content panel to full allele-level sequencing. The report should state what level was tested.

A gene-content test lists genes as present or absent. It may include inhibitory genes such as KIR2DL1, KIR2DL2, KIR2DL3, KIR2DL5, KIR3DL1, and KIR3DL2; activating genes such as KIR2DS1, KIR2DS2, KIR2DS3, KIR2DS4, KIR2DS5, and KIR3DS1; and framework or pseudogenes.

KIR haplotypes are commonly grouped as A or B. Group A haplotypes have a relatively fixed gene pattern dominated by inhibitory receptors, with KIR2DS4 as the main activating gene. Group B haplotypes are more variable and contain one or more B-defining genes, often including KIR2DL2, KIR2DL5, KIR2DS1, KIR2DS2, KIR2DS3, KIR2DS5, or KIR3DS1.

Because most routine tests examine the combined genotype rather than separating the two chromosome copies, reports often use:

• AA: two group A haplotypes are inferred;
• B/x: at least one group B haplotype is present;
• B-content score: a count or classification based on centromeric and telomeric B motifs;
• centromeric A/B and telomeric A/B motifs: regional haplotype content.

Report type	What it can show	What it may miss
Gene presence/absence	Which KIR genes were detected	Allele function, exact copy number, and phase
Copy-number testing	Zero, one, two, or more copies of selected genes	Full sequence and expression
Allele-level typing	Specific KIR variants	Actual receptor expression on each cell
KIR plus HLA ligand analysis	Potential receptor-ligand combinations	Clinical outcome certainty

Some alleles are poorly expressed, retained inside the cell, or encode truncated proteins. For example, common KIR2DS4 deletion variants may not produce a typical cell-surface receptor. A simple “gene present” result may therefore overstate functional activity.

Sample collection and laboratory methods

DNA can be collected from blood, saliva, or a cheek swab. No fasting is required. Medicines, pregnancy, infection, exercise, and time of day do not change inherited KIR genes, although they may alter natural killer cell numbers or activity in separate functional tests.

Laboratories use sequence-specific primer PCR, sequence-specific oligonucleotide probes, quantitative PCR, multiplex methods, next-generation sequencing, or long-read sequencing. The KIR region is difficult to analyze because its genes are highly similar and frequently duplicated, deleted, or rearranged. A method that works well for gene content may not resolve alleles or phase.

Before ordering, ask:

1. Is the test for gene content, copy number, alleles, or all three?
2. Does it include HLA-C1/C2 and Bw4 ligand typing?
3. Does the report classify AA versus B/x or calculate B-content?
4. Is it validated for the intended clinical use?
5. Can the laboratory distinguish common null or low-expression variants?

A natural killer cell count, cytotoxicity assay, or flow-cytometry receptor panel is not the same as KIR genotyping. Cell tests can change with illness and treatment; the genotype remains fixed. Someone can carry a KIR gene without expressing it on all natural killer cells.

Clinical samples should use identity safeguards, especially if the result could affect donor choice. Direct-to-consumer data are not adequate for transplant selection unless confirmed in a specialist laboratory.

Interpreting KIR with HLA ligands

KIR and HLA are inherited on different chromosomes and vary independently. A receptor may be present without its usual ligand, and a ligand may be present without the matching receptor. Interpretation therefore requires both sides.

HLA-C alleles are commonly divided into C1 and C2 groups according to an amino acid at position 80. People may be C1/C1, C1/C2, or C2/C2. HLA-B and selected HLA-A alleles may carry the Bw4 epitope; others carry Bw6, which is not a standard KIR3DL1 ligand.

Examples of descriptive results include:

• KIR2DL1 present with an HLA-C2 ligand;
• KIR2DL3 present with an HLA-C1 ligand;
• donor KIR3DL1 present while the recipient lacks Bw4;
• activating KIR2DS1 present in a donor with a recipient C2 ligand;
• KIR B/x genotype with high B-content.

These combinations describe biological possibilities, not a guaranteed response. Receptor expression varies among natural killer cell subsets. KIR alleles differ in binding and surface expression. HLA expression, peptide content, disease, conditioning, immunosuppressive drugs, and other receptors influence the actual effect.

A “ligand mismatch” can mean different things in different publications. Some models examine whether donor natural killer cells possess an inhibitory KIR for an HLA ligand missing in the recipient. Others infer KIR from donor HLA, use receptor-ligand pairs, or consider directionality. Reports should name the model rather than simply say matched or mismatched.

The HLA-C genotype is especially relevant because C1 and C2 are major KIR ligand groups. Broad HLA typing and KIR testing answer related but distinct questions.

Use in stem cell transplantation

KIR testing has been studied most extensively in allogeneic hematopoietic cell transplantation. Donor natural killer cells reconstitute early after transplant and may attack leukemia cells, influence graft-versus-host disease, control infection, and support engraftment.

Several donor-selection models have been proposed:

• inhibitory KIR ligand mismatch;
• donor KIR gene presence or absence;
• KIR B-content or B-motif selection;
• specific activating KIR, such as KIR2DS1;
• donor KIR allele and recipient HLA combinations;
• natural killer cell education or “missing ligand” models.

Results have varied across leukemia type, graft source, donor relationship, conditioning, T-cell depletion, graft-versus-host disease prophylaxis, and study design. A model associated with lower relapse in one setting may not reproduce in another.

KIR is therefore usually a secondary or research-level donor factor, not a replacement for established priorities. Transplant teams first consider HLA match, donor-specific antibodies, donor age, availability, disease urgency, graft source, and the planned prophylaxis. In selected programs, KIR may help choose among otherwise similar donors, particularly for acute myeloid leukemia or haploidentical transplantation.

A favorable KIR model cannot make an unsafe donor appropriate, and an unfavorable model does not automatically exclude a donor. Recent studies emphasize that prediction remains uncertain and that high-resolution KIR data may improve future models but are not yet universally standardized.

For natural-killer-cell therapies, KIR-HLA combinations can inform research on alloreactivity and product design. These uses involve specialized cellular therapy teams and differ from routine consumer immune testing.

Pregnancy, disease associations, and research uses

KIR on uterine natural killer cells and HLA-C on placental trophoblast cells help regulate placentation. Research has linked certain maternal KIR and fetal HLA-C combinations with preeclampsia, fetal growth problems, and reproductive outcomes. These are group-level associations with complex biology.

Commercial tests sometimes market KIR genotyping for infertility, implantation failure, or recurrent miscarriage. Evidence does not support using one KIR result alone to diagnose the cause, predict an individual pregnancy, or choose immune treatment. Maternal genotype, fetal paternal HLA contribution, obstetric history, anatomy, chromosomes, hormones, clotting disorders, and many other factors may matter.

KIR variation has also been studied in viral infections, cancer, autoimmune disease, inflammatory disorders, and response to therapy. Association results can conflict because KIR and HLA frequencies differ by population, sample sizes are often small, and many gene-ligand combinations are tested.

A result such as KIR AA is not an immune deficiency. B/x is not proof of excessive immune activation. Both are common normal patterns. KIR genes are part of normal population diversity.

Testing may be useful within a clinical study or when a specialist has a defined, evidence-based question. It is usually unhelpful as a broad explanation for recurrent infections, fatigue, infertility, or inflammatory symptoms. A primary immunodeficiency genetic panel evaluates a different category of inherited immune disorders when clinical features support that concern.

Limitations and common mistakes

KIR interpretation is limited by structural complexity, incomplete allele resolution, uncertain phase, variable expression, and lack of universal clinical thresholds. Gene presence does not equal receptor function. Genotype does not measure natural killer cell count or activity.

Common mistakes include:

• labeling activating genes as always beneficial or harmful;
• interpreting KIR without HLA-C1/C2 or Bw4 ligands;
• assuming AA is abnormal or B/x is superior;
• using a reproductive association as a diagnosis;
• selecting a transplant donor from KIR alone;
• comparing studies that use different mismatch models;
• treating one odds ratio as a personal probability;
• repeating the genotype to monitor treatment;
• confusing KIR genes with HLA genes.

Population diversity matters. A genotype that is uncommon in one ancestry group may be common in another. Studies may not transfer directly between populations. Multiple testing can also produce chance associations unless results are independently validated.

A laboratory report may not identify which genes lie together on each chromosome. Family segregation or long-read sequencing may be needed for phase. Even allele-level sequence does not reveal which receptors are expressed on a person’s current natural killer cell subsets.

Next steps after a KIR result

Obtain the full gene and allele list, assay method, resolution, and HLA ligand results. Ask why the test was ordered and which decision it is intended to support.

For transplantation, the transplant physician and histocompatibility laboratory should interpret KIR only after core donor factors. Ask whether the center uses a validated KIR model for the specific disease, graft source, and prophylaxis. Do not delay a necessary transplant while pursuing a theoretical KIR advantage without specialist guidance.

For pregnancy concerns, review the result with a maternal-fetal medicine or reproductive specialist who can explain the limits. Avoid unproven steroids, intravenous immunoglobulin, lipid infusions, anticoagulants, or other immune treatments solely because of KIR genotype.

For infection or immune symptoms, use standard clinical evaluation. Blood counts, immunoglobulins, vaccine responses, lymphocyte subsets, infection history, and targeted genetics may be more relevant than KIR.

Genetic counseling can explain inheritance and why relatives may have different gene-content patterns. The result generally needs to be tested only once unless a higher-resolution clinical method is required.

Questions to ask when KIR is being used clinically

A KIR report becomes more useful when the ordering team states the decision in advance. For a transplant candidate, ask whether KIR will break a tie between donors who are otherwise similar or whether the center has a disease-specific protocol. Ask which model is being used, because “KIR mismatch” may refer to donor HLA ligand status, directly typed donor receptors, missing recipient ligands, B-content, or a particular receptor-ligand pair. A conclusion from one model cannot automatically be substituted for another.

The transplant report should identify whether the donor or recipient was tested, whether HLA ligands were directly typed at adequate resolution, and whether allele-level KIR information was available. It should also state the disease and transplant platform for which the interpretation is intended. Evidence developed in T-cell-depleted haploidentical transplantation may not apply to an unrelated donor transplant using post-transplant cyclophosphamide.

When reproductive testing is proposed, ask what validated action follows each possible result. A test has limited clinical utility when a positive pattern has no agreed diagnostic meaning and no treatment proven to improve live-birth outcomes. Couples should also ask whether fetal HLA-C was measured or merely inferred, because maternal KIR alone cannot define the receptor-ligand combination at the placenta.

Why two people with the same KIR genes can differ

Gene-content results simplify a much richer system. Natural killer cells do not all express the same receptors. Each person has many NK-cell clones, and each clone expresses a different combination of KIR and other receptors. Infection, age, pregnancy, transplant conditioning, cytokines, and medications can reshape the proportions of these subsets without changing DNA.

Alleles also matter. Two people who both carry KIR3DL1 may express different amounts of receptor or have proteins with different binding strength for Bw4. The Bw4 ligand itself varies, including differences at position 80, and the peptide inside the HLA molecule can alter recognition. Similar complexity applies to HLA-C1 and C2 interactions.

Natural killer cells integrate KIR with NKG2A, NKG2D, CD16, cytokine receptors, and many other signals. Antibody-coated targets may activate CD16 even when inhibitory KIR are present. A tumor may lose one HLA molecule while increasing another. These layers explain why a simple activating-versus-inhibitory count cannot predict immune behavior.

A functional question may require flow cytometry, cytotoxicity testing, cytokine assays, or direct study of a cell-therapy product. Those tests answer different questions and can vary over time. Genotyping is best viewed as a map of possible receptors, not a live readout of what every NK cell is doing.

Privacy and family implications

KIR data can reveal biological relationships when combined with broader genetic information, although it is not normally used as a paternity test. Results may also be useful in future research as new receptor-ligand models emerge. Patients should understand how the laboratory stores data, whether deidentified results may be used for research, and how reanalysis is handled.

Relatives do not necessarily share the same KIR genotype. Each child receives one KIR haplotype from each parent, and structural variation creates many combinations. A sibling’s “favorable” donor result cannot be assigned to another sibling without testing. When a donor is selected, the clinical laboratory should use the donor’s own validated sample.

A final interpretation should state confidence. A directly sequenced receptor-ligand pair with a defined protocol carries more weight than an inferred haplotype from a small association study. When evidence is preliminary, the report should say so plainly. This prevents a common problem in immune genetics: converting an interesting biological association into a treatment rule before clinical benefit has been demonstrated.

People should also distinguish research eligibility from proven care. A KIR pattern may qualify someone for a study of donor selection or natural-killer-cell therapy without predicting that the experimental approach will help. Study participation can advance knowledge, but consent should explain alternatives, uncertainty, and which costs or follow-up are involved.

The result should be revisited only when a new clinical use emerges or a higher-resolution assay can answer a question the original test could not. Routine annual retesting wastes resources because inherited gene content remains stable. Instead, preserve the original report, method, sample identity, and HLA ligand data so future specialists can reinterpret them as evidence improves.

The ordering specialist should document whether the finding changes care now, may become relevant later, or is research-only. That simple distinction prevents unnecessary follow-up and unsupported treatment.

References

• Donor KIR genotype based outcome prediction after allogeneic stem cell transplantation: no land in sight 2024
• Systematic evaluation of donor-KIR/recipient-HLA interactions for HCT outcomes 2023
• The role and novel use of natural killer cells in graft-versus-leukemia reactions 2024 (Review)
• Impact of KIR-HLA Genotype on Natural-Killer-Cell-Based Immunotherapy in Solid Tumors 2024 (Review)
• The Role of Killer Ig-like Receptors in Diseases from A to Z 2025 (Review)
• IPD-KIR Database 2026

Disclaimer

This article provides general information about KIR genetics and does not diagnose immune disease, infertility, pregnancy risk, or transplant compatibility. KIR results should be interpreted by the relevant specialist with HLA ligand data and the full clinical context. Do not choose a donor or start immune treatment based only on a KIR report.