Which blood group system's D antigen is used to determine transfusion compatibility?

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The D antigen is the defining marker of the Rh blood group system and is the main determinant used to assess transfusion compatibility. In transfusion medicine, matching Rh status (Rh positive if D antigen is present, Rh negative if it is absent) helps prevent alloimmunization. The D antigen is highly immunogenic, so an Rh-negative recipient who receives Rh-positive blood can develop anti-D antibodies. Those antibodies can cause a hemolytic transfusion reaction with future exposures and, in pregnancy, can lead to hemolytic disease of the fetus and newborn. Serologic testing uses anti-D antisera to determine D presence and label blood as Rh positive or negative, guiding safe donor–recipient matching. Other antigen systems like MN or Kell exist, but the D antigen within the Rh system is the key factor for determining transfusion compatibility.