I was immune most of my life, too - and camped out and did a ton of outdoor things. Never broke out.
Then I had it in my yard without knowing it. Laid down in the grass with the side of my face on it to take a picture - didn’t break out for 3 days, then nearly had to be hospitalized.
It lasted 6 weeks and was very severe, on my arms and face. I had cortisone shots, the oral dosepacks, topical ointments. Nothing *really* helped except time, and then educating myself.
But each time I’ve had a tiny exposure, it has been a bit less - but then, I’m usually prepared now - and I know not to scratch any strange itches, even if they seem like just mosquito bites.
Plain *cold* running water works for me, when it’s not a contact skin poison. I have a recurring hand rash now that’s giving me fits, but it’s something in my system, not outside.
Oh, also, I found out that once you get the poison ivy allergy, you also shouldn’t eat cashews or mangoes, as they are cousins and have something similar. My doc said he felt my severe allergy built up from exposure after exposure mowing my yard every week, until finally I became severely allergic - the opposite of some allergies that diminish with more exposure.