That’s interesting...
When I was in my 30’s, worked in my backyard clearing foliage. Was never afflicted with poison ivy. Ended up with the worst case my dad (a physician), had ever seen ! Weeping lumps as big as a fist all over my body. He said it was overexposure. Now I’m highly susceptible.
It’s similar to how HIV patients exposed to a higher viral load will experience intense symptoms and an initial high viral load
A common misconception about poison ivy (and similarly bee venom) is that people will build up resistance with exposure. Quite the opposite is true; recurring exposure increases sensitivity and severity of reaction.