The first Kell system antibody was described in 1946 by Mrs. Kellacher, which was later also found in Mrs. Cellano. The Kell and Cellano blood groups are symbolized K and k, respectively. The KEL gene is present on chromosome 7 and it has been cloned and characterised. At present this blood group system comprises 22 blood group antigens some showing a distinct racial prevalence (K antigen is more frequently found in Northern European, the Jsa antigen is most frequently found in those of African descent and the Kpc antigen has been more frequently found in Japanese). Different Kell phenotypes result from point mutations leading to amino acid changes in the Kell glycoprotein. The Kell blood group protein is a zinc endopeptidase and Kell is expressed primarily in the erythroid tissues, and in a near-equal amount in testis, with weaker expression in a large number of other tissues.
The McLeod (K null) phenotype was described in a man of that surname whose red cells showed weak reactivity to Kell antisera. The precursor missing in McLeods red cells is called Kx whose absence is associated with a set of clinical symptoms that include nerve & muscle disorders, red cell acanthocytosis and chronic granulomatous disease.
The Kell blood group system is important in transfusion medicine because the Kell antigen is second only to Rh (D) antigen as a cause of haemolytic disease of the newborn in alloimmunised women. If a Kell negative woman carries a Kell positive baby, the baby’s red cells can sensitise the mother who develops antibodies to them. Subsequently, these antibodies in the mother can cause red cell breakdown in the baby leading to severe anaemia and jaundice. A simple PCR assay has been developed to determine the Kell type of an individual and can determine the Kell status of fetal material. This is made use in the clinical management of pregnancies in which the fetus is at risk for haemolytic disease of the newborn (HDN) due to anti-K.