A dermatologist friend told me there are people who react to poison ivy, and people who do not yet react to poison ivy. He explained to the effect that repeated exposure tends to sensitize people. Poison ivy/oak/etc. has no effect on them, then one day it does.
I grew up playing in woods thick with the stuff and never had a problem. Then I had a hell of a reaction to it in high school, and it’s been cruel to me since then.
This article in the WSJ was by an author who decided to induce immunity to poison oak by exposure: eating it. It's... quite the journey, evidently, but possible:
Congratulations, you’ve encountered someone with passion. It lives in people all over the world, but Japanese craft has a particular gift for making it visible in every detail, every material, and every decision pushed to its absolute perfection. Toyotas quality, Grand Seikos polish and accuracy and Jiros sushi just to mention a few of the exported ones.