# Kell antigen system
The Kell antigen system is a group of 38 red blood cell surface antigens encoded by the KEL gene, one of the clinically important human blood group systems after ABO and Rhesus. Its defining antigen, K (Kell, ISBT KEL1), is strongly immunogenic, and antibodies against Kell antigens cause hemolytic transfusion reactions and hemolytic disease of the fetus and newborn (HDFN).[1](https://www.ncbi.nlm.nih.gov/books/NBK609095/) • [2](https://blooddatabase.isbtweb.org/database)
| Key fact | Detail |
|---|---|
| ISBT designation | Symbol KEL, system number 006; 38 recognized antigens[1](https://www.ncbi.nlm.nih.gov/books/NBK609095/) • [2](https://blooddatabase.isbtweb.org/database) |
| Discovery | 1946, by Coombs, Mourant, and Race; named for Mrs Kelleher, whose anti-K antibody caused HDFN in her newborn[3](https://www.ncbi.nlm.nih.gov/books/NBK2270/?report=reader) • [4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3595629/) |
| Main antigens | K (KEL1) and k (KEL2, Cellano) are antithetical and differ by a single amino acid, Thr193Met; the K−k+ phenotype occurs in 98% of Blacks and 91% of Caucasians[3](https://www.ncbi.nlm.nih.gov/books/NBK2270/?report=reader) • [5](https://www.genecards.org/card/KEL) |
| Gene | KEL on chromosome 7 (GRCh38 chr7:142,941,113–142,962,363), 19 exons, encoding a 732-amino-acid type II transmembrane glycoprotein (CD238)[2](https://blooddatabase.isbtweb.org/database) • [6](https://www.isbtweb.org/asset/E2E59D08-7BC6-4B66-887B105E7652C4E5/) |
| Protein function | A zinc-dependent metalloproteinase with endothelin-3-converting enzyme activity, joined by a single disulfide bond (Kell Cys72–XK Cys347) to the XK membrane protein that carries the Kx antigen[6](https://www.isbtweb.org/asset/E2E59D08-7BC6-4B66-887B105E7652C4E5/) |
| Clinical weight | Immunogenicity third after ABO and Rhesus D; anti-K is implicated in severe hemolytic transfusion reactions and HDFN[1](https://www.ncbi.nlm.nih.gov/books/NBK609095/) |
| Fetal expression | Kell antigens are detectable on fetal erythrocytes as early as 10 weeks gestation[1](https://www.ncbi.nlm.nih.gov/books/NBK609095/) |
## Discovery and history
The system was described in 1946, only weeks after the introduction of the antiglobulin (Coombs) test. Coombs, Mourant, and Race investigated cases of maternal-fetal incompatibility and found a mother's serum that reacted with her husband's red cells and with about 7 to 9 percent of random donor samples but fit no known system; Race named the antigen "Kell" after the mother's surname.[3](https://www.ncbi.nlm.nih.gov/books/NBK2270/?report=reader) • [7](https://www.ojp.gov/redirect-legacy/ncjrs/pdffiles1/pr/160880_unit_5.pdf) • [8](https://www.sciencedirect.com/topics/nursing-and-health-professions/blood-group-kell-system) The antithetical antigen k, called Cellano, was identified in 1949 as a high-frequency antigen, and the system remained a simple two-antigen system until 1957, when the antithetical Kpa and Kpb antigens and the K0 (Kell-null) phenotype were reported.[8](https://www.sciencedirect.com/topics/nursing-and-health-professions/blood-group-kell-system) The Jsa and Jsb antigens, found in 1962 and 1963, were initially assigned to a separate "Sutter" system before being folded into Kell.[9](https://www.ojp.gov/pdffiles1/Digitization/94227NCJRS.pdf)
## Genetics and biochemistry
The KEL gene sits on chromosome 7 and encodes the Kell glycoprotein, a single-pass type II membrane protein of 732 amino acids, also known as CD238. Most Kell antigens arise from single nucleotide polymorphisms producing single amino acid substitutions; the K/k distinction is a Thr193Met change, and each of the other antigens similarly traces to a defined point mutation rather than a partial-expression pattern.[6](https://www.isbtweb.org/asset/E2E59D08-7BC6-4B66-887B105E7652C4E5/) • [5](https://www.genecards.org/card/KEL) • [1](https://www.ncbi.nlm.nih.gov/books/NBK609095/)
Kell is a zinc-dependent metalloproteinase of the M13 family, related to neprilysin, and shows endothelin-3-converting enzyme activity. A single disulfide bond links it to the XK membrane protein, which carries the Kx antigen; absence of XK reduces Kell antigen expression on the red cell membrane.[6](https://www.isbtweb.org/asset/E2E59D08-7BC6-4B66-887B105E7652C4E5/) • [5](https://www.genecards.org/card/KEL) This linkage ties the Kell system to the XK (ISBT 019) and Gerbich (ISBT 020) systems.[10](https://onlinelibrary.wiley.com/doi/full/10.1111/voxs.12593)
**Phenotype frequencies.** The k antigen is common in nearly all populations: the K−k+ phenotype is found in 98% of Blacks and 91% of Caucasians, so K-positive individuals are the minority, and anti-K serum reacts with roughly 7 to 9 percent of random donor samples.[3](https://www.ncbi.nlm.nih.gov/books/NBK2270/?report=reader) • [7](https://www.ojp.gov/redirect-legacy/ncjrs/pdffiles1/pr/160880_unit_5.pdf) The system also contains many high-prevalence antigens (such as Ku/KEL5, KEL11 through KEL14, KEL18, KEL19, and others) and low-prevalence antigens (such as Kpa/KEL3, Jsa/KEL6, Ula/KEL10).[1](https://www.ncbi.nlm.nih.gov/books/NBK609095/) Rare K0 red cells lack all Kell glycoprotein and every Kell system antigen.[11](https://www.sciencedirect.com/topics/medicine-and-dentistry/blood-group-kell-system)
## Antibodies and laboratory behavior
Anti-K antibodies are usually IgG, do not bind complement, and mostly react at 37 °C in the anti-human globulin (AHG) phase; IgM examples can react at room temperature, and naturally occurring anti-K occasionally appears, including transient examples in patients with [Escherichia coli](https://www.edgepedia.org/escherichia-coli) infections that disappear after recovery.[1](https://www.ncbi.nlm.nih.gov/books/NBK609095/) Anti-K reactivity can be depressed with some low-ionic-strength solution reagents, so AHG-phase testing is required for reliable detection.[1](https://www.ncbi.nlm.nih.gov/books/NBK609095/) Kell antigens resist treatment with proteolytic enzymes such as ficin, papain, and chymotrypsin, but all Kell antigens (together with the Cartwright Yta antigen) are denatured by dithiothreitol (DTT) in the same concentration range, a property linked to disulfide bond integrity that laboratories exploit when distinguishing Kell from other antibody specificities.[1](https://www.ncbi.nlm.nih.gov/books/NBK609095/)
## Clinical significance
**Transfusion.** The K antigen is the system's clinically dominant member; one overview notes that of the more than 34 antigens, only K1 matters in routine practice.[11](https://www.sciencedirect.com/topics/medicine-and-dentistry/blood-group-kell-system) Anti-K has been implicated in severe hemolytic transfusion reactions and HDFN, and antibodies to high-prevalence Kell antigens such as k create the practical problem that few antigen-negative donor units exist, even though antibody formation against such antigens is rare because of their prevalence.[1](https://www.ncbi.nlm.nih.gov/books/NBK609095/) People with the K0 null phenotype can make anti-Ku (anti-KEL5) after transfusion, an antibody capable of causing mild to severe transfusion reactions, with at least one fatal case reported.[12](https://www.ncbi.nlm.nih.gov/books/NBK2270/pdf/Bookshelf_NBK2270.pdf) • [13](https://www.ncbi.nlm.nih.gov/sites/books/NBK609095/)
**Pregnancy.** Because Kell antigens appear on fetal red cells from about 10 weeks gestation, anti-Kell antibodies found in a pregnant person are followed closely to prevent and manage hemolytic disease of the fetus and newborn.[1](https://www.ncbi.nlm.nih.gov/books/NBK609095/) Non-invasive fetal Kell genotyping on circulating fetal DNA, using allelic discrimination of K1 and K2, has been developed as a testing approach.[1](https://www.ncbi.nlm.nih.gov/books/NBK609095/)
**McLeod syndrome.** Loss of the XK protein produces [McLeod syndrome](https://www.edgepedia.org/mcleod-syndrome), an X-linked recessive multisystem disorder with neurological, cardiovascular, and hematologic features, in which Kell antigen functions on red cells are lost; the Kell system is also associated in the literature with chronic granulomatous disease.[1](https://www.ncbi.nlm.nih.gov/books/NBK609095/)
## Nomenclature
The International Society of Blood Transfusion (ISBT) assigned the system symbol KEL and number 006; each antigen carries a six-digit number in which the first three digits identify the system, for example 006003 for Kpa, also written KEL3.[1](https://www.ncbi.nlm.nih.gov/books/NBK609095/) • [14](https://www.isbtweb.org/isbt-working-parties/rcibgt/blood-group-terminology.html) Phenotypes are written with a colon and comma-separated antigens, absent antigens marked with a minus sign (for example KEL:−1,2), and alleles in italic form such as KEL*02.[14](https://www.isbtweb.org/isbt-working-parties/rcibgt/blood-group-terminology.html) The ISBT allele table records eight antithetical antigen pairs within the system, including KEL1/KEL2, KEL3/KEL4/KEL21, and KEL6/KEL7.[6](https://www.isbtweb.org/asset/E2E59D08-7BC6-4B66-887B105E7652C4E5/)
## References
1. [Kell Blood Group System, StatPearls, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK609095/)
2. [ISBT Blood Group Database](https://blooddatabase.isbtweb.org/database)
3. [The Kell blood group, Blood Groups and Red Cell Antigens, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK2270/?report=reader)
4. [A Brief History of Human Blood Groups, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3595629/)
5. [KEL Gene, GeneCards](https://www.genecards.org/card/KEL)
6. [(ISBT 006) KEL blood group alleles v8.0, 30 September 2023](https://www.isbtweb.org/asset/E2E59D08-7BC6-4B66-887B105E7652C4E5/)
7. [Sourcebook in Forensic Serology, Immunology, and Biochemistry, Unit 5](https://www.ojp.gov/redirect-legacy/ncjrs/pdffiles1/pr/160880_unit_5.pdf)
8. [Blood Group Kell System: an overview, ScienceDirect Topics](https://www.sciencedirect.com/topics/nursing-and-health-professions/blood-group-kell-system)
9. [Ss, Kell, Duffy and Kidd Antigens, NCJRS](https://www.ojp.gov/pdffiles1/Digitization/94227NCJRS.pdf)
10. [Blood group systems (Smart, 2020), ISBT Science Series](https://onlinelibrary.wiley.com/doi/full/10.1111/voxs.12593)
11. [Blood Group Kell System: an overview, ScienceDirect Topics (medicine and dentistry)](https://www.sciencedirect.com/topics/medicine-and-dentistry/blood-group-kell-system)
12. [The Kell blood group (PDF), NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK2270/pdf/Bookshelf_NBK2270.pdf)
13. [Kell Blood Group System, StatPearls (alternate site)](https://www.ncbi.nlm.nih.gov/sites/books/NBK609095/)
14. [Blood Group Terminology, ISBT Working Party](https://www.isbtweb.org/isbt-working-parties/rcibgt/blood-group-terminology.html)
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*Topic: Encyclopedia › Life and health › Human health and medicine › Human structure and function › Cardiovascular and lymphatic systems › Blood groups and transfusion medicine*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license