Anti-c can be formed by persons with which genotype?

• A. R^1 R^2
• B. R^2 R^2
• C. r r
• D. R^1 R^1

Answer: (D)

Anti-c antibodies form in people who do not express the c antigen on their red cells. The RHCE gene haplotype known as R1 (often described as DCe) carries the C antigen and e, but not the c antigen. So a person with two copies of that haplotype (R1R1) would be negative for the c antigen. If such an individual is exposed to cells that do have the c antigen, their immune system can form anti-c. Other genotypes listed include haplotypes that provide the c antigen, so they would express c and are unlikely to form anti-c. Therefore, the R1R1 genotype best explains the possibility of anti-c formation.