Kidd blood group system: outwardly simple with hidden complexity

The basics of the Kidd blood group system have been well described. Two of the three antigens, Jka and Jkb, are polymorphic. The third, Jk3, is of very high prevalence in all populations except in individuals of Polynesian and Finnish extraction. Jk (a−b−) red cells do not express Jk3. The antigens are encoded by SLC14A1 gene on chromosome 18. JK*01 allele differs from JK*02 at nt 838 (G/A) and in the protein, at amino acid 280, Jka expression is associated with aspartate and asparagine is associated with Jkb. The genetic background of the dominantly inherited In(Jk) Jk(a−b−) is due to an 84 bp deletion in ZFN850 on chromosome 19. Antibodies against all three antigens are clinically important. They can cause immediate haemolytic transfusion reactions and are a frequent cause of delayed transfusion reactions. Haemolytic disease of the foetus and newborn due to JK antibodies is generally much less severe than the response to incompatible transfusion. The JK protein on red cells functions as a urea transporter. Complexity in the system is due to the number of alternate alleles described. ISBT recognizes 24 alleles that silence Jka/Jkb expression and 7 alleles causing weak or partial expression of Jka or Jkb. These alleles can result in serological typing discrepancies, discordant serological/molecular results and production of an apparent Kidd alloantibody by an antigen‐positive individual. JK antigens may serve as minor histocompatibility antigens. The antibodies may play a role in renal graft survival when a JK‐incompatible donor kidney is transplanted.