Other Blood Group Systems

OTHER BLOOD GROUP SYSTEMS

The Lewis blood group system (ISBT 007, symbol Le) is related the Lewis antigens Le^a and Le^b present on erythrocytes. However, these antigens are not integral parts of the membrane but are soluble plasma proteins which become reversibly adsorbed onto erythrocyte membranes. The levels of bound antigens therefore vary, although the erythrocytes of children 2 years old and above have approximately adult levels.

The Le^a and Le^b antigens are not the products of different forms of a single gene, but arise from different actions of a fucosyl transferase that attaches fucose residues to an oligosaccharide known as type-1 precursor oligosac-charide. If the fucose is added to a subterminal position it produces the Le^a antigen, whereas attachment to the terminal position gives the Le^b antigen. Approximately 72% of white populations are Le(a–b+), that is, they lack the Le^a but have the Le^b, 22% are Le(a+b–), and 6% lack both antigens. Antibodies to the Le antigens are usually of the IgM class and, as such, do not cause HDN since they do not cross the placenta.

The Duffy system of blood group antigens (ISBT 008, symbol FY) is comprised of six antigens, of which Fy^a and Fy^b are the most significant in transfusion reactions. These antigens are expressed on an erythrocyte membrane glyco-protein and also form the site of attachment for malarial parasites . Thus a Fy(a–b–) individual, who does not express the blood group antigens, has a selective advantage in a malarial area. Indeed, 68% of blacks of African descent are of this phenotype, which is rare in whites. Antibodies to Duffy antigens belong to the IgG class and may cause HDN.

The Kidd antigens (ISBT 009, symbol JK) are expressed on a membrane glyco-protein, which is associated with urea transport. The Jk^a and Jk^b antigens result from the expression of a codominant pair of genes. Approximately 27% of whites, and 57% of blacks are Jk(a+ b–) while 50% of whites and 34% of blacks are Jk(a+ b+). The Jk(a–b+) phenotype is found in 23% of whites and only 9% of blacks, while the Jk(a–b–) phenotype is rare in both populations. Antibodies to Jk^a and Jk^b belong to the IgM or the IgG class and may cause a mild form of HDN.

The Kell blood group system (ISBT 006, symbol KEL) is formed from 24 anti-gens expressed on a glycoprotein of the erythrocyte membrane. The antigen K (formerly Kell) is highly immunogenic and IgM or IgG antibodies to it are common in transfused patients. Similarly, antibodies to its allele, designated k (formerly Cellano), can also cause HDN, although specific antibodies arerare in transfused patients.