Among Rh system antigens, which is most immunogenic?

• A. C
• B. E
• C. c
• D. D

Answer: (D)

The strongest immune response in the Rh system comes from the D antigen. When a D-negative individual is exposed to D-positive red cells, the immune system is highly likely to recognize D and mount a robust alloantibody response, typically IgG. This high immunogenicity stems from the D antigen being expressed densely on the red cell surface and presenting multiple epitopes that easily trigger helper T and B cell activation. Clinically, anti-D is the most frequently formed Rh antibody and is the primary cause of hemolytic disease of the fetus and newborn when the fetus is D-positive and the mother is D-negative. The other Rh antigens (C, c, E) can provoke antibodies as well, but they are less likely to provoke a strong immune response compared with D.