Off the top of my head, here is a link to a past discussion on the topic of natural treatments for poison ivy, oak & sumac.
Note that Christopher suggests Sassafras can be used both internally (drunk as a tea) and externally - topically applied to the rash. There was another poster in this thread who mentioned that Yellow Dock is good BECAUSE it influences the body's iron content; translation - those who regularly suffer from poison ivy/oak/sumac is because of a deficiency in the way their body handles iron. I don't doubt this may be true in my case, but I am a big meat eater, which at least by conventional wisdom, is supposed to be a good source of nutritional iron..........not sure what this implies. Anyway, you may find some help from reading the various posts that started at this link: