Facts about the M and N Antigens and Antibodies:
- Part of the MNS System.
- M and N antigens are located on the red blood cell surface glycoprotein, glycophorin A.
- Anti-M and Anti-N can be naturally occurring or can be immune stimulated. A naturally occurring antibody occurs without stimulus related to a transfusion or pregnancy.
- Anti-M is considered clinically insignificant in most cases and does not typically cause acute or delayed hemolytic transfusion reactions. The antibody is rarely associated with HDFN.
- Anti-M either is IgG or has an IgG component along with the IgM component between 50-80% of the time. The antibody reacts best at room temperature or 4οC.
- When Anti-M is reacting at IAT only and is in the IgG form of the antibody rather than the IgM form, or has dual components, hemolysis can occur in these cases. Therefore, it is recommended that M-negative units be transfused temporarily until the Anti-M drops in titer and stops demonstrating at IAT.
- Units for transfusion are typically crossmatch compatible at the IAT phase and do not require M-negative donor cells. Anti-M may react in gel IAT, therefore performing the crossmatch in tube IAT may be best to find compatible units.
- In patients undergoing procedures that include induced hypothermia, M-negative crossmatch-compatible units at IS and IAT may be recommended at some institutions.
- When Anti-M occurs in pregnancy, it needs to be determined if the antibody is IgM or IgG. If it is determined that the Anti-M is an IgG antibody, prenatal titration studies will be performed throughout the pregnancy. Antibody levels rarely reach a high enough titer to affect the fetus. The newborn should be monitored for anemia during the neonatal period.
- The most practical treatment option is to provide M-negative blood if transfusion is indicated. The availability of M-negative blood is usually not an issue.
- Anti-N is not considered clinically significant and may be associated with patients on dialysis treatment. However, those individuals with an N-S-s- phenotype that make an extremely clinically significant Anti-N that causes severe HTRs.
- Anti-M and Anti-N are sensitive (destroyed) to enzyme treatment.
Table 7. M and N Facts.| Ag/Ab | Ag Frequency (White) | Ag Frequency (Black) | Dosage | Enzyme Interaction | Antibody Class | Complement Binding | Clinically Significant |
| M | 78% | 74% | Yes | Destroyed by | IgM, IgG | No | No |
| N | 72% | 75% | Yes | Destroyed by | IgM, IgG* | No | No |
*Anti-N is rarely IgG.
