Two other antigens were found on the surface of human red blood cells. M and N. Analyzing the blood of several people, it was found that in some there was only the antigen M, in others only the N and several people had both antigens. It could then be concluded that there were three groups in this system: M, N and MN.
The genes that condition the production of these antigens are only two: L M and L N (The letter L is the initial of the discoverer, Landsteiner). This is a case of simple Mendelian inheritance. The genotype L ML M, conditions the production of antigen M, and L NL N, the antigen N. Between L M and L N there is co-dominance, so people with genotype L ML N produce both types of antigens.
|M||L ML M|
|N||L NL N|
|MN||L ML N|
MN System Transfusions
Antibody production anti-M or anti-N occurs only after awareness (you will see this in the HR system). Thus, there will be no incompatibility reaction if a person belonging to group M, for example, receives N-type blood unless he or she is sensitized by previous transfusions.