A patient positive for weak D also shows anti-D in his serum. What could be the issue?

In this scenario, the presence of both weak D and anti-D in the patient’s serum indicates a complex immunological situation. When a patient presents as weak D, it typically means they express a reduced amount of the D antigen on their red blood cells. In some cases, individuals with weak D typing can produce anti-D antibodies if they are exposed to the D antigen from a transfusion or pregnancy, particularly if the person’s weak D phenotype results from an inherited variation known as a D mosaic. This mosaicism can lead to the absence or altered expression of certain epitopes of the D antigen, which might prompt the immune system to recognize the surrounding antigenic areas as foreign, resulting in the production of anti-D antibodies.

While other factors related to testing errors or disease states could influence results, the presence of anti-D in a weak D patient is most closely associated with the presence of a D mosaic. These individuals are at risk of forming antibodies against the epitopes they do not express, thereby making option D the most accurate explanation for the observed scenario.