I just learned I'm allergic to sulfa drug the hard way a couple weeks ago. Never had that reaction to any antibiotic before. So, I'll pass on eating poison oak, if that's ok. But I know the idea has merit. There was a king in the Ancient World named Mithridates who made himself immune to several poisons by ingesting tiny doses of them over time. He also had the rather interesting hobby of growing poisonous plants and feeding poisons to prisoners to test out antidotes. Unfortunately for Mithridates, his immunity to poisons caught up with him one day. He ran afoul of the Romans and needed to commit suicide to avoid the embarrassing fate Romans traditionally reserved for captured leaders (look what happened to Vercingetorix.) He tried to take poison, but none of it had any effect on him due to his immunity. So, he came to fairly messy end at the point of a sword and a decidedly painful one.
