Policies for administering RhIg when the mother's Rh type is weak D vary among countries and within some countries.
To understand the issues involved, we need to review the genetics of the Rh D antigen and the types of weak D (formerly Du). The Rh system is complex and only a basic overview will be given.
In brief, Rh system inheritance is determined by two sets of genes:
- RHD codes for the proteins carrying D expression. In most people, the presence or absence of the RHD gene results in being Rh positive or Rh negative, respectively.
- RHCE codes for different combinations of the proteins carrying CcEe expression and Rh-associated glycoprotein (RhAG). RhAG is needed for the expression of Rh antigens. The RHCE locus is adjacent to RHD on chromosome #1.
- See a model for the Rh locus (NCBI)
For a small percentage of people, the inheritance of the RHD gene and the expression of the D antigen may be altered leading to several variants of D encoded by more than 100 RHD alleles