Clinical Significance of the Identification of an Anti-M (IgG) Alloantibody in Pregnancy.

Anti-M is usually a naturally occurring, cold-reactive immunoglobulin M (IgM) antibody and is commonly regarded as clinically insignificant. An immunoglobulin G (IgG) component, however, can cross the placenta and has rarely been associated with hemolytic disease of the fetus and newborn (HDFN), including hyporegenerative fetal or neonatal anemia. A 42-year-old woman (G4P2A2) undergoing preconception evaluation had a positive indirect antiglobulin test (IAT) with initially inconclusive antibody identification. Reference immunohematology testing identified anti-M reactivity at 4°C. After dithiothreitol (DTT) treatment, an immunoglobulin G component was confirmed at a titer of 1:2. The direct antiglobulin test (DAT) remained negative, and there was no evidence of maternal hemolysis. Her partner was M antigen positive, indicating that a future fetus could inherit the target antigen. This case shows that detailed characterization of an apparently cold-reactive anti-M antibody can materially change preconception counseling and pregnancy planning. Low titer alone should not replace consideration of antibody class, thermal amplitude, obstetric history, paternal or fetal antigen status, and coordinated follow-up by transfusion medicine and maternal-fetal medicine teams.