MNS System
The antithetical M and N antigens are carried on glycophorin A and encoded by GYPA on chromosome 4. The homologous GYPB encodes the antithetical S and s antigens that are carried on glycophorin B. Similar to the Rh blood type, multiple recombinations can occur between GYPA and GYPB creating numerous variants.
Antibodies against M and N may occur naturally and are usually of an IgM isotype that only reacts at cold temperatures and is therefore not clinically significant. In contrast, antibodies against S and/or s tend to be provoked by sensitization, are of an IgG subtype that reacts at 37ºC and have been implicated in hemolytic reactions.