Which antibody is associated with genotype R1R1?

• A. Anti-D
• B. Anti-C
• C. Anti-E
• D. Anti-c

Answer: (D)

In this Rh system context, antibodies form against antigens that are not present on your own red cells. The R1R1 genotype corresponds to two identical haplotypes that carry the C and D antigens but do not have the c antigen on either chromosome, so the red cells express C and D (and usually e) but lack the c antigen. If this person is exposed to red cells carrying the c antigen, their immune system can recognize that missing antigen and produce anti-c antibodies. Anti-D would not be expected here because the individual is D positive, and anti-C would target the C antigen which is present on their own cells. Anti-E targets the E antigen, which is also absent on their cells, but anti-c is the antibody most directly associated with lacking the c antigen in this genotype.