Anti-c (Little c) IgM: An Uncommonly Observed but Expected Phenomenon

The c-antigen (little c) is part of the Rh blood group system and is found in approximately 80% of the United States population. Anti-c antibody develops in individuals sensitized through previous exposure and is associated with acute and delayed hemolytic transfusion reactions as well as hemolytic disease of the newborn (HDN). Most antibodies produced against Rh antigens are of the immunoglobulin (Ig) G type. We present a case of anti-c in a 30-year-old white woman who was gravida 2 para 1 (G2P1), whose laboratory workup at the time of admission for delivery suggests recent exposure and seroconversion in the latter part of her pregnancy, with evidence of the expected but rarely demonstrated presence of anti-c IgM and IgG.