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Transfusion considerations concerning patients with anti-hrB and confounding co-morbid conditions: pregnancy and coagulopathy.

Alloimmunisation to hematopoietic antigens is generally considered to arise from three sources: pregnancy, blood transfusion, or organ transplantation. The most common red blood cell (RBC) antigens of clinical significance are those of the ABO and Rhesus (Rh) blood group systems. The Rh blood antigens comprise two linked genes showing a high degree of homology. For historical reasons, the genes are named RHD and RHCE. RHD encodes the ubiquitous RhD protein, for which only one common allele exists, whose absence in Rh-negative individuals elicits a strong antigenic response when challenged by exposure to allogeneic RhD-positive RBC. The well-known alloimmunisation reaction can lead to frank complement-mediated haemolysis in individuals with a past history of exposure; in pregnancy, this can lead to haemolytic disease of the foetus and newborn, or in severe cases, erythroblastosis foetalis. By contrast, at the linked RHCE locus, two intragenic antigenic determinants exist and each has a pair of alleles, designated C, c, E, and e1–4. While conventional variants at these loci do not usually elicit a strong immune response, rare polymorphic alleles at these loci may result in haemolytic disease of the foetus and newborn when allogeneic RBC challenge occurs. 
 
One infrequent allele present in African-Americans of South African or Caribbean descent is designated hrB or Rh315. The hrB variant is thought most commonly to result from three amino acid changes in the RHCE allele that appear to confer a dominant allointolerance to common RHCE alleles6,7. The antigen conferring sensitivity in hrB-negative individuals has recently been more extensively studied by molecular characterisation of RHCE haplotypes in alloimmune individuals. This molecular analysis has revealed molecular heterogeneity among genetically susceptible individuals carrying the associated (C)ceS haplotype8. 
 
Features of haemolysis in offspring of hrB mothers are minimal5 and hrB is generally thought not to cause clinical sequelae of haemolytic disease of the foetus and newborn3; nevertheless, avoidance of alloimmunisation is recommended. As hrB-negative units are extraordinarily rare, family matches or Rhnull units from rare donor pools are often solicited. We report two instances of hrB alloimmunity, one in a multiparous patient with a twin pregnancy and the other in a male with sickle cell disease and coagulopathy.