RhD antigen:

Unlike the ABH antigen system, which is composed of carbohydrates and to which naturally occurring antibodies develop routinely in early childhood, the Rh system is composed of polypeptides, and antibodies usually occur only following exposure to a foreign antigen (ie., via transfusion or pregnancy).

The most important antigen within the Rh system is the RhD. 85% of the Caucasian population are RhD positive. For most RhD negative individuals, the RHD gene is deleted, while in many RhD negative individuals with African ancestry, a pseudogene encoding a 37-base pair duplication results in a stop coding preventing expression of the RhD antigen. There is no minor allele of the RhD gene, meaning that when an RhD-negative individual is exposed to RhD-positive RBCs, they are being exposed to a wide variety of antigenic epitopes and are therefore at high risk of being sensitized.

Once an anti-D has formed, it may cause hemolytic transfusion reactions and hemolytic disease of the fetus and newborn. Great care is therefore taken to avoid sensitizing RhD-negative individuals with RhD-postive erythrocytes. In pregnant women who may be carrying an RhD-positive fetus, this involves the administration of anti-D immune globulin (also known as RhIg) during pregnancy and at delivery. For RhD-negative transfusion recipients, it requires the avoidance of RhD-positive RBC transfusions.

Next page: RhD Typing

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