Weak D alleles express distinct phenotypes

The weak D phenotype is caused by many different RHD alleles encoding aberr ant RhD proteins, raising the possibility of distinct serologic phenotypes and of anti-D immunizations in weak D. We reported 6 new RHD alleles, D cat egory III type IV, DIM, and the weak D types 4.1, 4.2.1, 4.2.2, and 17. The immunohematologic features of 18 weak D types were examined by agglutinati on and flow cytometry with more than 50 monoclonal anti-D. The agglutinatio n patterns of the partial D phenotypes DIM, D-III type IV, and D-IV type II I correlated well with the D epitope models, those of the weak D types show ed no correlation. In flow cytometry, the weak D types displayed type-speci fic antigen densities between 70 and 4000 RhD antigens per cell and qualita tively distinct D antigens, A Rhesus D similarity index was devised to char acterize the extent of qualitative changes in aberrant D antigens and discr iminated normal D from all tested partial D, including D category III. In s ome rare weak D types, the extent of the alterations was comparable to that found in partial Ds that were prone to anti-D immunization. Four of 6 case reports with anti-D in weak D represented auto-anti-D. We concluded that, in contrast to previous assumptions, most weak D types, including prevalent ones, carry altered D antigens, These observations are suggestive of a cli nically relevant potential for anti-D immunizations in some, but not in the prevalent weak D types, and were used to derive an improved transfusion st rategy in weak D patients. (Blood, 2000;95:2699-2708) (C) 2000 by The Ameri can Society of Hematology.