Mechanism 2: Weak D

In this mechanism, the individual has inherited an RHD gene that codes for a weakened expression of the D antigen. The D antigen seems to be complete but there are fewer numbers of the antigen on the cell surface. Based on molecular studies, it has been determined that mutations in the RHD gene occur, causing changes in amino acids present in the trans-membrane or intracellular areas of the RhD protein. Recipients of D positive blood who possess the weak D phenotype rarely form anti-D likely because the changes in the protein are "inside" the cell membrane, are complete yet fewer in number. As many as 53 mutations of this nature have been identified. Detection of the D antigen in individuals of the weak D phenotype may require indirect antiglobulin testing, though many of the newer monoclonal anti-D reagents are capable of detecting it during immediate spin phase. It is critical to consult the manufacturer's direction circular regarding the use of the reagent in determination of D status in recipients and donors as the clones used may react differently and some are not suitable for use in the IAT method. When making transfusion decisions regarding individuals who are weak D, the institutional policy should be followed.