Cold reacting anti-M causing delayed hemolytic disease of the newborn

BACKGROUND: Hemolytic disease of the fetus and newborn (HDFN) is due to passively transferred maternal antibodies directed against fetal red blood cell (RBC) antigens and can lead to severe morbidity and mortality. Anti-M is usually a naturally occurring antibody of low clinical significance, although occasionally severe cases of HDFN are seen. CASE REPORTS: Two M+ sisters are presented, each developing hemolysis during the first 2 weeks of life due to maternal anti-M, resulting in severe anemia and requiring blood transfusion. RBC agglutination was observed in peripheral blood samples of both infants at room temperature with dissociation at 37°C. Maternal anti-M detected by column indirect agglutination technique, was of low titer (1:16) and demonstrated low thermal amplitude, reacting in saline at 4°C but was not detectable in saline at 37°C. CONCLUSIONS: Anti-M of low thermal amplitude may cause hemolytic disease of the newborn with laboratory features resembling cold agglutinin disease