A study many many-years ago on infants who had never been exposed to poison ivy (in an institution for developmentally disabled) showed that if they were fed poison ivy extract directly into their stomachs, they would develop a life-long tolerance.

In Japan, China, Korea, the Japanese lacquer tree has the same allergenic oil (urushiol) with subtle chemicals differences that make it a good medium to create the famous lacquerware. The lacquer painters can come to the occupation with sensitivity, but most over time lose it, and when they leave temporarily for a vacation for example, they might eat some of the urushiol paint while gone.

Nobody should eat a leaf of poison oak/ivy on a whim though, because you could get a raging head to toe case.

ps, most articles on the internet on poison oak/ivy/sumac have many wrong statements, because the author copied from other wrong articles instead of finding clinical studies.

My credentials: I wrote “The Poison Oak & Poison Ivy Survival Guide.”