Anti-M antibodies are mostly IgM, less commonly mixture of IgM and IgG and rarely solely of IgG type. The naturally occurring Anti-M is usually inactive at 37°C making it insignificant clinically. In less common cases the antibody is active at 37°C making it clinically significant due to its ability to cause haemolytic transfusion reactions (HTR) and haemolytic disease of fetus and newborn (HDFN). We report a case of naturally occurring Anti-M antibody active at 22°C and 37°C as well, detected incidentally in a nulliparous never transfused woman. This clinically significant antibody caused discrepancy in blood grouping also. M antigen negative red cells are required for transfusion in such cases and its presence creates a potential risk of HDFN in future pregnancies.
Keywords: Anti-M antibody, Red cell alloantibody, Naturally occurring anti-M.