Kidd Blood Group System
The Kidd blood group system was discovered in 1951. It was named in recognition of Mrs. Kidd, a patient who was found to have produced antibodies targeted against some antigens on the RBCs of her foetus, resulting in fatal haemolytic disease in the newborn.
The Kidd blood group system consists of three known antigens- Jka, Jkb, and Jk3. The antigens were named ‘Jk’ after the initials of Mrs. Kidd’s baby- John Kidd.
INTRODUCTION TO KIDD ANTIGENS
Figure: Structure of Kidd Glycoprotein
The Kidd antigens, also known as Jk antigens, are located on a multi-pass glycoprotein (that passes through the RBC membrane ten times) made up of 389 amino acids. The Kidd glycoprotein, also known as XK protein, functions to maintain the osmotic stability of RBCs by acting as a urea transporter.
The Jka and Jkb antigens differ by a point mutation, which results in an aspartic acid (Asp) to asparagine (Asn) change at amino acid 280. The expression of Asp on the amino acid position 280 leads to the formation of the Jka antigen. In contrast, the expression of Asn leads to the formation of the Jkb antigen.
However, unlike the Jka and Jkb antigens, the Jk3 antigen is not associated with a specific amino acid substitution at a particular position. The Jk3 antigen is a universal antigen that represents the presence or absence of Kidd glycoprotein on the surface of red blood cells.
INHERITANCE OF KIDD ANTIGENS
All three antigens (Jka, Jkb, and Jk3) of the Kidd blood group system are encoded by the SLC14A1 gene (solute carrier family 14, member 1) located on Chromosome 18. The SLC14A1 gene has two major co-dominant alleles JKa and JKb, and the recessive silent allele JK.
JKa: expression of JKa allele leads to the formation of Jka and Jk3 antigens.
JKb: expression of JKb allele leads to the formation of Jkb and Jk3 antigens.
JK: expression of JK allele leads to the absence of Jka, Jkb, and JK3 antigens.
Humans are diploid organisms that contain two copies of an allele, one inherited from each parent. Therefore, an individual can either inherit:
at least one copy of JKa (JKa/JKa or JKa/JK)
at least one copy of JKb (JKb/JKb or JKb/JK)
one copy of JKa along with one copy of JKb (JKa/JKb)
two copies of the silent allele JK (JK/JK)
Figure: Inheritance of Antigens in Kidd Blood Group System
The possible Kidd phenotypes are Jk (a+b-), Jk (a+b+), Jk (a-b+), and Jk (a-b-). The JK3 antigen is present in all individuals except the rare phenotype Jk (a-b-), which lacks the Kidd glycoprotein on the surface of their RBCs.
Jk null Phenotype
The antigen Jk3 is present in nearly 100% of all populations. The rare phenotype Jk (a-b-), also known as the ‘Jk null phenotype’ does not contain Jka, Jkb, or Jk3 antigens. Individuals with the Jk null phenotype can produce anti-Jk3 antibodies and cause haemolysis of the donor RBCs that contain either Jka, Jkb, or JK3 antigens.