Kidd antigen system

Human blood group classification
The Kidd antigen system (also known as Jk antigen) are proteins found in the Kidd's blood group, which act as antigens, i. e., they have the ability to produce antibodies under certain circumstances. The Jk antigen is found on a protein responsible for urea transport in the red blood cells and the kidney. They are important in transfusion medicine. People with two Jk(a) antigens, for instance, may form antibodies against donated blood containing two Jk(b) antigens (and thus no Jk(a) antigens). This can lead to hemolytic anemia, in which the body destroys the transfused blood, leading to low red blood cell counts. Another disease associated with the Jk antigen is hemolytic disease of the newborn, in which a pregnant woman's body creates antibodies against the blood of her fetus, leading to destruction of the fetal blood cells. Hemolytic disease of the newborn associated with Jk antibodies is typically mild, though fatal cases have been reported. The gene encoding this protein is found on chromosome 18. Three Jk alleles are Jk (a), Jk (b)and Jk3. Jk (a) was discovered by Allen et al. in 1951 and is named after a patient (Mrs Kidd delivered a baby with a haemolytic disease of the newborn associated with an antibody directed against a new antigen Jk (a). Whereas Jk (b) was discovered by Plant et al. in 1953, individuals who lack the Jk antigen (Jk null) are unable to maximally concentrate their urine.

Kidd comprises three antigens on a glycoprotein with 10 transmembrane spanning domains, cytoplasmic N and C termini and one extracellular N glycosylation site. The Kidd gene has 11 exons with exons 4 11 encoding the mature protein. The Kidd gene (SLC14A1) is on chromosome 18q12.3.

Jka and Jkb are the products of alleles with Asp280 and Asn280 in the fourth external loop of the Kidd glycoprotein. Jka and Jkb have similar prevalences in White and Asian populations but Jka is more common in Black populations than Jkb.

Kidd antibodies display dosage: red cells from homozygous individuals (JkaJka or Jk(a+b )) express more antigen than heterozygous individuals (JkaJkb or Jk(a+b+)). Anti Kidd antibodies appear to react more strongly against cells that are homozygous.

Kidd antibodies can be difficult to detect by direct agglutination testing and generally require addition of antihuman globulin after a warm incubation period.

Kidd antibodies are dangerous as they are capable of causing severe acute hemolytic transfusion reactions. They are unique in that they are capable of dropping to low or even undetectable levels after several months following an exposure. As red blood cells approach the renal medulla (where there is a high concentration of urea), the urea transporter allows for rapid uptake of urea and prevents cell shrinkage in the hypertonic environment of the medulla.