Which antigen is most commonly tested for in the Rh system?

• A. C
• B. E
• C. D
• D. c

Answer: (C)

In Rh typing, the presence of the D antigen is what defines Rh positivity, and it is the most immunogenic of the Rh antigens. Because D is highly capable of provoking an antibody response, detecting it reliably identifies the clinically important Rh status: individuals who have D are Rh positive, those without D are Rh negative. This is crucial since anti-D antibodies can cause severe hemolytic disease of the newborn and adverse transfusion reactions, so labs routinely test for D to guide safe transfusion and prenatal care. Other Rh antigens like C, E, and c exist and may be typed in extended phenotyping, but they are less routinely used to determine Rh status and have lower immunogenicity.