MNS system

The MNS blood group system is second only to the Rh blood group system in its complexity. Many alloantibodies to antigens in the MNS system are not generally clinically significant--these are often “naturally occurring” and if they do not react at 37°C can be ignored in transfusion practice. However, rare examples of anti-M, anti-N, anti-S, -s, and antiU have been reported to cause immediate and delayed hemolytic transfusion reactions and hemolytic disease of the fetus and newborn (HDFN). Anti-M is often cold-reacting IgG and is enhanced by testing in an acidified environment.