Which statement is true about the D antigen in transfusion compatibility?

• A. It is present on Rh positive cells.
• B. It is absent from Rh positive cells.
• C. It is present on all A and B antigens.
• D. It defines the Bombay phenotype.

Answer: (A)

Understanding that D antigen defines Rh status is essential. The D antigen is a protein on the surface of red blood cells; when it is present, the person is Rh-positive, and when it is absent, the person is Rh-negative. This distinction matters in transfusions because an Rh-negative recipient exposed to Rh-positive blood can form anti-D antibodies, leading to a hemolytic transfusion reaction or, in pregnancy, to hemolytic disease of the newborn. The D antigen operates independently of ABO antigens, which are carbohydrate structures determining A and B types. The Bombay phenotype is defined by lack of H antigen, not by the D antigen, making it a separate issue from Rh typing. Therefore, the statement that D antigen is present on Rh positive cells is the correct description.