Kidd antigen system

The Kidd antigen system (also known as Jk antigen) is present on the membranes of red blood cells and the kidney and helps determine a person's blood type. The Jk antigen is found on a protein responsible for urea transport in the red blood cells and the kidney. The gene encoding this protein is found on chromosome 18. Two common Jk alleles are Jk(a) and Jk(b). Individuals who lack the Jk antigen (Jk null) are unable to maximally concentrate their urine.

The Jk antigen is 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. HDN associated with Jk antibodies is typically mild, though fatal cases have been reported.