Unusually Detected Anti-M Antibody Presenting as Cross Match Incompatibility in a Female Child Diagnosed with Small Round Cell Tumour.

MNS antigen system is one of the human blood group systems. Anti-M antibody is a relatively common, naturally occurring antibody of IgM variety. Clinically significant anti-M antibody is reactive at 37°C in the anti-human globulin phase due to high thermal amplitude of IgM component or presence of IgG component. If anti-M antibody is activated at 37°C or in the anti-human globulin phase, it may cause delayed haemolytic transfusion reactions or haemolytic disease of newborn, which suggest variable clinical significance. We report a case of an unusually detected anti-M antibody presenting as cross match incompatibility in a one-year-old female child with a lump in the right lumbar region, which was later diagnosed as small round cell tumour in the right kidney.