The Kell antigen system (Kell–Cellano system, K, k, Kp, Kx antigen) is a group of antigens on the human red blood cell that characterise a blood group important in haemolytic disease of the newborn and a potential cause of severe autoimmune haemolytic anaemia if either the Anti-K or AntiKu antibodies are generated. It was first recognised in 1946 shortly after the introduction of the antiglobulin test. Two alleles, KEL1 (Kell, K1, Met-193) and KEL2 (Cellano, K2, Thr-193), are the most common variants of the KEL gene at 7q34 which encodes the 732 amino acid pro-peptide of Kell blood group glycoprotein, a type II transmembrane glycoprotein that is highly polymorphic. At least 25 variants are known, The membrane transport protein XK, which has other important functions, is linked via a single disulfide bond to the expressed Kx antigen. The encoded protein is a zinc endopeptidase with relatively specific endothelin-3-converting enzyme activity. Active endothelin-3 is a potent vasoconstrictor. It is often simpler to characterise the weaker KEL antigens by polymerase chain reaction genotyping which also rationalises the confusion of names somewhat.
|Some characterised KELL groups||Amino acid change||Other names|
|K0||No protein but multiple causes|
|K||Met-193||KEL1, Kell, K1, Kell (named after Mrs. Kellacher)|
|k||Thr-193||KEL2, K2, Cellano (named after Mrs. Cellano)|
Absence of the Kell protein (K0), usually results on blood exposure to the formation of the anti-Ku antibody which is capable of causing a mild to severe transfusion reaction. So such individuals must be transfused with blood from donors who are also K0. There is an association due to gene linkage of the K0 phenotype and loss of ability to taste phenylthiocarbamide. K0 phenotype does not produce Mcleod syndrome.