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.