Lower antigen site density and weak D immunogenicity cannot be explained by structural genomic abnormalities or regulatory defects of the RHD gene

The weak D phenotype is characterized serologically by a weak or negative agglutination reaction with polyclonal anti-D in an immediate-spin test. Agglutination is enhanced in the indirect antiglobulin test. Red cells that are typed weak D have a much lower number of apparently complete D antigens at their cell surface and are associated with considerably weaker immunogenicity than are red cells with normal D. In a previous study, the number of D sites per cell was determined in eight unrelated weak D individuals to range from 490 to 1870 D sites per cell, which corresponded to 4 to 14.2 percent of the number of D sites in CcDee samples. The RHD gene was investigated for structural abnormalities by Southern blot experiments and polymerase chain reaction-based RHD typing in these individuals. In addition, abnormalities in the transcription process were studied by sequence analysis of RH transcripts and by comparing the relative amounts of RHD mRNA in weak D to those in CcDee, CcDEe, and -D- samples by using a semiquantitative reverse transcriptase-polymerase chain reaction analysis. The RHD gene in weak D phenotypes does not show any abnormalities at either the genomic or the transcriptional level when compared to the RHD gene in normal D phenotypes. The weaker immunogenicity of weak D is not explained by structural difference in the RHD gene itself. The weaker expression of D might be caused by factors involved in the Rh-related complex or by an as yet unidentified suppressor gene. This study supports the concept that weak D phenotypes carry complete D polypeptides and reflect a quantitative rather than a qualitative variation of D