Kidd system

The Kidd blood group system (Jk) is defined by two alleles, Jka and Jkb (frequency 0.51 and 0.48 in Europeans). There are three common phenotypes Jk(a+b−), Jk(a−b+) and Jk(a+b+) and a rare null phenotype, Jk(a−b−). Antibodies that target Kidd antigens are a significant cause of delayed hemolytic transfusion reactions. Anti-Kidd antibodies are also a cause of hemolytic disease of the fetus and newborn (HDFN).