Transfusion medicine

Red blood cell antigens

Kell system

Editorial Board Member: Kyle Annen, D.O.
Deputy Editor-in-Chief: Patricia Tsang, M.D., M.B.A.
Brian D. Adkins, M.D.
Garrett S. Booth, M.D., M.S.

Last author update: 11 February 2021
Last staff update: 12 January 2024

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PubMed Search: Transfusion[TIAB] Kell[TI] group

Brian D. Adkins, M.D.
Garrett S. Booth, M.D., M.S.
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Cite this page: Adkins BD, Booth GS. Kell system. website. Accessed May 19th, 2024.
Definition / general
  • Highly immunogenic
  • Antibodies clinically significant
Essential features
  • Most Kell antigens are widely expressed on the surface of red blood cells in the donor population
  • Antibodies can cause hemolysis and hemolytic disease of the fetus and newborn (HDFN)
  • Kell is expressed in early erythroid precursors, which can lead to suppression of erythropoiesis in the fetus
  • Type: peptides found within the Kell protein (CD238); the Kell blood group antigens are expressed on a large transmembrane protein with extensive disulfide bonds; the function of this protein is unknown (AABB: Technical Manual, 19th Edition, 2017)
    • 36 antigens total (AABB: Technical Manual, 19th Edition, 2017)
    • K or KEL1, often referred to erroneously as Kell antigen, is highly immunogenic but has low prevalence; it is encoded by the KEL gene on chromosome 7 (AABB: Technical Manual, 19th Edition, 2017)
    • k or K2 (archaic Cellano), is a high prevalence antigen; it is a codominant allele and can be expressed alongside KEL1 (AABB: Technical Manual, 19th Edition, 2017)
    • Many Kell antigens are expressed in high frequency amongst the donor population, specifically k, Jsb, Kpb and Ku; K (KEL1), is expressed less frequently and often matched in sickle cell disease patients due to its high immunogenicity
    • Kell antigens are destroyed by dithiothreitol treatment, which is commonly used when evaluating panels for patients on daratumumab; as such, patients on daratumumab must undergo phenotyping prior to drug administration or genotyping in order to provide Kell matched red blood cell products, as antibodies cannot be identified by routine methods
    • Kx is a protein associated with Kell antigens required for normal expression on red cells; absence of Kx, which is encoded by the XK gene, leads to decreased expression of Kell antigens, acanthocytosis and the clinical presentation of McLeod phenotype (AABB: Technical Manual, 19th Edition, 2017)
    • Kell null phenotype arises in individuals inheriting defective KEL gene (K0) and may form Ku antibodies; Kell null red cells have no expression of Kell antigens but increased levels of Kx

Table 1: prevalence of KEL phenotypes
Race / Ethnicity
White 91 8.8 0.2 97.7 2.3 Rare 100 Rare 0
African American (US) 98 2 Rare 100 Rare 0 80 19 1
Adapted from AABB: Technical Manual, 19th Edition, 2017
  • Majority IgG, fewer IgM (AABB: Technical Manual, 19th Edition, 2017)
  • Occurs with transfusion, pregnancy or activities such as intravenous drug use
  • Can cause hemolytic reactions and hemolytic disease of the fetus and newborn
  • Kell antigens are expressed in early erythropoiesis so there is immune clearance of red cell precursors; this leads to severe anemia without significant fetal hemolysis
  • Kell null individuals can develop antibodies against Ku, a high frequency Kell antigen and subsequently require blood from other K0 (Kell null) individuals
  • Not generally affected by dosage and unaffected by enzyme treatment
  • K or KEL1
  • k or K2 (archaic Cellano)
Clinical features
  • Exposure to Kell antigens secondary to pregnancy or transfusion
Case reports
Board review style question #1
Which group of antigens is expressed early in erythropoiesis, leading to hemolytic disease of the fetus and newborn without significant hemolysis?

  1. Duffy
  2. Kell
  3. Lutheran
  4. Rh
Board review style answer #1
B. Kell. Erythropoiesis is inhibited in the fetus by binding of Kell antigens, which are expressed on early erythroid precursors. (N Engl J Med 1998;338:798)

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Reference: Kell group
Board review style question #2
Kell antigens have a high prevalence in the donor population. Which Kell antigen is expressed by almost all donors?

  1. K1
  2. K2
  3. Kpa
  4. Ku
Board review style answer #2
D. Ku. Ku or universal is expressed by the majority of the donor population. Of note, many Kell antigens are high frequency antigens such as k, Jsb and Kpb. K (KEL1) is less frequently expressed (~9%) but highly immunogenic.

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Reference: Kell group
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