The last decade has witnessed an abundance of information detailing the genetic diversity of the RH locus which has exceeded all estimates predicted by serology. Well over 120 RHD and over 60 different RHCE alleles have been documented, and new alleles are still being discovered. For clinical transfusion medicine, RH genetic testing can now be used to determine RHD zygosity, resolve D antigen status, and detect altered RHD and RHCE genes in individuals at risk for producing antibodies to high-incidence Rh antigens, particularly patients with sickle cell disease (SCD).T ransfusion of patients with sickle cell disease (SCD) represents a significant challenge in clinical transfusion medicine with red blood cell (RBC) alloimmunization a primary and serious complication of transfusions. A major cause of alloimmunization in patients with SCD is the disparate distribution of red cell antigens between donors, who are primarily of European ancestry, and patients with SCD, who are primarily of African ancestry. Management of alloimmunization in SCD has been the subject of much debate, 1,2 and currently there is no standard approach. Over two thirds of alloantibodies formed by patients with SCD have Rh blood group specificities. Many programs transfuse patients with SCD with RBCs that are phenotype-matched for D, C/c, E/e, and K, and some also supply RBCs from African-American donors when possible. Although these approaches reduce the incidence of alloantibody production, patients still become alloimmunized. Genetic analysis of patients who develop antibodies in the face of conventional antigen matching has revealed that these patients carry altered RH alleles.
RH Genes and Rh ProteinsTwo genes, RHD and RHCE, lie in close proximity on chromosome 1 and encode 416-amino acid Rh proteins: one encodes the D antigen, and the other encodes CE antigens in various combinations (ce, cE, Ce, or CE) (Figure 1). Each gene has ten exons; they are 97% identical and encode proteins that differ by 32 to 35 amino acids (Figure 1). This contrasts with most blood group antigens, which are encoded by single genes with alleles that differ TRANSFUSION MEDICINE ___________________________________________________________________________________ by only one or a few amino acids. For comprehensive reviews, see Westhoff 3 and Avent. The conventional RH genes shown in Figure 1 are found in all population groups, although with different frequencies. Commercial antibody reagents detect expression of the five principal Rh antigens: D, C, c, E, and e. Variant RHD and RHCE alleles encode Rh proteins with amino acid changes that cannot be distinguished serologically, but can be recognized by the immune system as foreign. Of relevance for transfusion in patients with SCD, RH alleles encoding altered C and e antigens are frequent in African ethnic groups. 5,6 Some patients with SCD are at risk for production of antibodies to both the RhCE and RhD proteins, a serious and potentially life-threatening complication.
Variations of RhD