RhD

RhD is the D antigen of the Rh blood group system. It is expressed on the surface of red blood cells and is used to classify blood as RhD positive or RhD negative. The D antigen is highly immunogenic; individuals lacking D antigen can form anti-D antibodies after exposure to D-positive blood, which can cause hemolytic transfusion reactions or, in pregnancy, hemolytic disease of the fetus and newborn (HDFN).

Genetics and variants: The D antigen is encoded by the RHD gene on chromosome 1. A functional RHD gene yields D antigen expression, while deletion or inactivation results in RhD negative status. Several variants exist, including weak D and partial D, which can complicate serologic testing and transfusion decisions.

Clinical significance: In transfusion medicine, matching for RhD is routine; RhD negative recipients are given RhD negative units when possible to avoid anti-D formation. Anti-D antibodies are typically IgG and can cross the placenta, potentially causing fetal hemolysis in RhD negative mothers carrying an RhD positive fetus.

Pregnancy management and prophylaxis: RhD immune globulin (RhIG) is administered to RhD negative pregnant women during pregnancy and after delivery if the newborn is RhD positive, to prevent sensitization and reduce the risk of HDFN.

Testing and diagnosis: Serologic typing determines D status; antibody screening with indirect antiglobulin tests helps detect alloantibodies. The direct antiglobulin test is used in newborns to diagnose HDFN. Molecular typing can identify RHD variants in complex cases.

Epidemiology: The frequency of RhD negativity varies by population, being around 15% in people of European descent and lower in many Asian populations, with considerable regional variation.