COMMENTARY Where are we in efforts to unravel the complexity of Rh to guide transfusion decisions? Sandra Taddie Nance and Christine Lomas-Francis “ T he Rh blood group system is the most complex and polymorphic blood group system and has great clinical relevance with regard to transfusion and hemolytic disease of the fetus and newborn (HDFN).” Papers discussing the Rh blood group system, almost invariably, open with a statement forewarning the uninitiated reader of the com- plexity of Rh. These complexities have long been recog- nized, initially in serologic testing and more recently as a result of gene interrogation. Serologically, the complexity referred to the plethora of antigens and the variable expression of some, also the unusual phenotypes, of which several expressed multiple low-prevalence or lacked high-prevalence antigens. The finding of anti-D in a D+ person, reported by Shapiro 1 in 1951 and Argall and colleagues 2 in 1953, was the first indication that partial antigens (in this case D) existed. In 1960, Shapiro 3 reported finding an apparent anti-e in an e+ patient and presaged the recognition of partial e antigens, but many years elapsed before partial C and c antigens were recog- nized. The precise serology of these early findings paved the way for recognition of the phenotypic variance of reac- tivity. The cloning of the RH genes 4,5 in the early 1990s and advances in molecular techniques opened the door to applying DNA-based testing. The DNA analysis revealed that serology had only detected the tip of the iceberg as far as the complexity of Rh genotypes. More than two decades have passed since the begin- ning of the era of blood group genomics. The field has been in the data gathering stage, vital for evidence-based decision making concerning patient care and making major operational changes that affect testing algorithms for transfusion decisions and blood supply. Hundreds of alleles encoding Rh proteins have been identified; many more than there are antigens or serologically distinguish- able phenotypes. Reactivity differences with monoclonal antibodies of the 1980s are explained by changes at the nucleotide level. For most blood groups, once the gene was cloned it was relatively simple to correlate differences in DNA sequences to antigen expression on the red blood cell (RBC). Unlike most other systems, prediction of Rh antigen expression through DNA typing is not simply a matter of identifying one or two single-nucleotide poly- morphisms. This is reinforced by the articles in this special edition of TRANSFUSION. Each addresses a facet of Rh, be it weak D, 6,7 partial D, 8,9 or D– phenotypes 10 or RHCE variant alleles, those that are hybrid alleles, 11 or those that lead to silencing of antigens 12,13 or to partial expression 14 or a survey of RHD and RHCE allele frequency in sub- Saharan Africa. 15 Ultimately, regardless of the topic of the article, each also has as one of its intended outcomes, the improvement of the practice of transfusion medicine, especially for some of the vulnerable patient groups. This collection of articles and two decades of Rh molecular experience provide the perfect opportunity to reflect on our current and future practices. CURRENT STATUS: 2013 DNA-based testing of patient’s samples is used to resolve complex antibody investigations, especially for those who appear to have made an antibody to an antigen expressed on their RBCs. Consider the patient with D+ C- E- c+ e+ RBCs whose serum reacts with all panel cells tested suggesting the presence of a warm autoantibody. Adsorption studies with selected allogeneic RBCs reveal the presence of anti-D, anti-E, and anti-e. Reflex RHD and RHCE gene interrogation showed homozygosity for RHCE*ceAR and RHD*DAR. This result predicts that D, c, and e are partial antigens [and a hr S - Hr- (RH:-18) phe- notype] and that the patient is C- and E- indicating that the anti-D, -E, and -e are likely to be alloantibodies. The sting in the tail is that the patient can also make alloanti-C, -c, and -f along with antibodies to common non-Rh anti- gens. In the past, the cross-matches for such patients From American Red Cross, Biomedical Operations and Northeast Division, Philadelphia, Pennsylvania; and Immunohematology, New York Blood Center, Long Island City, New York. Address reprint requests to: Sandra Nance, American Red Cross Blood Biomedical Services, Penn-Jersey Region, Philadelphia, PA 19123; e-mail: Sandra.Nance@redcross.org. Received for publication August 4, 2013; and accepted August 6, 2013. doi: 10.1111/trf.12406 TRANSFUSION 2013;53:2840-2843. 2840 TRANSFUSION Volume 53, November 2013