Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Rh and RhAG Blood Group Systems
The Rh blood group system (C, c, E, e, D, and more than 50 other antigens) is second only to ABO in clinical importance because the Rh antigens, especially D, are highly immunogenic and the antibodies can result in delayed hemolytic transfusion reactions (HTRs) and hemolytic disease of the fetus and newborn (HDFN). The RH locus consists of two homologous genes, RHD and RHCE, located in close proximity on chromosome 1 and encoding proteins consisting of 417 amino acids. The D-negative phenotype is associated with deleted or mutated/inactive RHD gene. The system is complex, especially in African black and Hispanic ethnic groups both serologically and genetically, as point mutation(s) and genetic exchange between the two genes generate new epitopes on the Rh proteins and novel antigens. While antisera is commercially available to detect the common D, C/c, and E/e antigens, serologic reagents are not available to identify the many other antigens; however, detection is possible by DNA methods, i.e., RH genotyping. The Rh system is associated with production of antibodies with multiple and complex specificities that can make it difficult to find compatible red blood cell (RBC) products. DNA testing to determine the patient’s RH genotype aids in component selection and transfusion management particularly for patients with sickle cell disease (SCD).
Common Antigens
The Rh system has more than 50 antigens, but those of routine concern are D, carried on RhD protein and encoded by the gene designated RHD , and C, c, E, and e antigens, carried on the RhCE protein encoded by the gene designated RHCE ( Table 26.1 ). Antigens are codominantly expressed. RHD and RHCE genes, each consisting of 10 coding regions (exons), are 97% homologous encoding proteins differing by between 32 or 35 amino acids. The two genes are inherited as an Rh haplotype. Prevalence of Rh haplotypes differs by ethnic group ( Table 26.1 ). A third gene, RHAG, is 47% identical in the coding region to RHD and RHCE and encodes the ancestral protein RhAG. Rh antigens are carried on hydrophobic 12-pass transmembrane proteins ( Fig. 26.1 ). RhAG is important for trafficking the RhD and RhCE proteins to the membrane, and lack of RhAG results in the absence of Rh antigen expression (Rh-null phenotype) or marked reduction of Rh antigen expression (Rh-mod phenotype). Rh and RhAG proteins form the Rh core complex in the membrane, and they interact with CD47, glycophorin B (carry SsU antigens), LW, and AE1 (also known as Band 3). This complex is linked to the membrane skeleton via Rh/RhAG–ankyrin interaction and CD47–protein 4.2 association. RhAG is involved in ammonia/ammonium transport and is important for cation balance in RBCs. Function of the Rh blood group proteins, RhD and RhCE is not known, but RBCs lacking all Rh antigens have structural abnormalities (i.e., stomatocytes). RhAG is a highly conserved protein but carries several antigens; two of high prevalence (RHAG1, RHAG3) and one of low prevalence (RHAG2).
Table 26.1
Nomenclature and Prevalence of Rh Haplotypes
Modified from Hillyer, C. D., Strauss, R. G., & Luban, N. L. C. (Eds.). (2004). Handbook of pediatric transfusion medicine . San Diego, CA: Elsevier Academic Press.
| Haplotype | Shorthand for Haplotype | Prevalence (%) | ||
|---|---|---|---|---|
| White | Black | Asian | ||
| DCe | R 1 | 42 | 17 | 70 |
| DcE | R 2 | 14 | 11 | 21 |
| Dce | R 0 | 4 | 44 | 3 |
| DCE | R z | <0.01 | <0.01 | 1 |
| ce | r | 37 | 26 | 3 |
| Ce | r′ | 2 | 2 | 2 |
| cE | r″ | 1 | <0.01 | <0.01 |
| CE | r y | <0.01 | <0.01 | <0.01 |
D Antigen
Presence or absence of the D antigen confers the Rh-positive or Rh-negative status commonly used in lay and scientific parlance. In the United States, ∼85% of the white population is D-positive (Rh-positive), and 15% is D-negative (Rh-negative), while only 5%–8% of blacks and less than 1% of Asians are D-negative. Most D-negative individuals, especially of European descent have a deletion of the RHD gene. D-negative phenotype can also occur as a result of various mutations in RHD , including premature stop codons, insertions, deletions, or RHD / RHCE hybrid alleles. The inactive RHD pseudogene, which results from a 17-bp duplication and premature stop codon, is frequently found in blacks. D-negative is rare in Asia, and D typing is not routinely done in some Asian countries.
Weak D
The term “serologic weak D” is applied to RBCs that carry low levels of D antigen (formerly called D u , which is an obsolete term, and its use is discouraged), which are not detected, or give a less than robust reactions by direct agglutination with anti-D. Weak D can be detected after incubation with anti-D and use of antiglobulin (AHG) reagent. However, if a patient has a positive DAT, weak D test will not be valid, as the test may be a false positive due to immunoglobulin (IgG) coating the cells. Because manufacturers’ reagents contain different clones and formulations, reactivity of weak D RBCs may be variable and depend on the specific reagent and the method used for detection. Alternatively, DNA testing can be used to accurately determine the D status.
More than 95 different RHD alleles account for the molecular basis of weak D expression, with weak D types 1, 2, and 3 being the most common in Caucasians. Weak D alleles most often have a single amino acid change. Serologic weak D can also result from decreased D antigen expression when RH ∗ C is in trans to RHD (phenotype Dce/Ce). A very weak form of D, termed D el , is only detected by adsorption and elution of anti-D and is more prevalent in Asians.
RBCs with weak D are less immunogenic than normal D-positive RBCs, but anti-D stimulation can occur. Components from individuals with weak D are labeled as D-positive, but not all weak D RBCs are detected by donor testing. It is important to investigate if a D-negative patient given an apparent D-negative product makes anti-D.
Patient samples do not need to be tested for weak D, with the exception of newborns, as the mother is a candidate for Rh immune globulin (RhIG) administration if the baby is weak D-positive. If testing for weak D is performed and found to be positive, or RBCs give less than a robust reaction with anti-D, further consideration should be given to assess the presence and risk of anti-D, especially in females of child-bearing potential. If the positive test is due to a weak D phenotype, the patient can generally receive D-positive RBCs without risk of immunization, but if the weak D reactivity is due to a partial D phenotype (below), then the patient is much more likely to become immunized against D. Serology reagents cannot differentiate a serologic weak D from a partial D phenotype, as both can present with weaker than expected reactivity with anti-D. RHD genotyping is recommended to determine the D status (below).
Partial D
Individuals with partial D (previously termed D mosaic) lack epitope(s) of the RhD protein, and when exposed to D antigen through transfusion or pregnancy can produce allo anti-D. There are numerous different partial D phenotypes. Routine D-typing reagents cannot distinguish partial D phenotypes. RBCs expressing partial D can give strong, variable, or weak reactions with anti-D, depending on the specific type of partial D. Patient typing is usually performed with an IgM monoclonal anti-D reagent that does not detect, in the direct phase, the partial DVI phenotype, which is the most common partial D in Caucasians. The most common partial D, DIIIa, in African-Americans usually goes undetected in serologic tests as the RBCs give strong reactions with anti-D. Many partial D phenotypes (also known as D categories) have arisen as a result of nucleotide exchange between the two genes RHCE and RHD , and less often from single-nucleotide changes in RHD .
Females of child-bearing potential who have a partial D phenotype benefit from receiving D-negative RBCs for transfusion and potentially benefit from RhIG prophylaxis. In practice, most are frequently typed as positive and are recognized only after they form anti-D. Importantly, DNA-based testing can distinguish these and should be considered in females of child-bearing potential, especially if they present with variable or weak RBC typing with anti-D reagents.
C/c and E/e Antigens
C and c differ by six nucleotide substitutions causing four amino acid changes. Only the Ser103Pro position strictly correlates with C/c antigenicity and expression of the C or c antigen, respectively. E and e differ by one nucleotide change, resulting in one amino acid difference, c.676C>G, p.Pro226Ala.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here