From Diablo Wiki
About Lower Resist:
I have played single game with PlugY,which can show your character's resistance detail(include magic resistance)
I obserbed Lower Resist casted by Oblivion Knight can reduce your magic resistance(Plug Y showed a reduce in magic resistance).But Lower Resist casted by player can not do this.
I obtained this information by casting Lower Resist on Greater Mummy,which can not break its magic immunity.