M antigen

The M antigen belongs to the MNS antigen system. Anti-M antibodies are usually found in the sera of persons who have had no exposure to human red cells. Although M antibodies are generally thought to be predominantly IgM, many examples that are partly or wholly IgG are frequently found. The antibodies are rarely clinically significant. However, in a few exceptional cases, anti-M detectable at the antiglobulin phase has caused hemolytic disease of the fetus and newborn (HDFN) or hemolysis of transfused cells.