I almost forgot. I had 2 friends that had a cure, they claimed. One made a tea with PI leaves each spring and drank it and the other chewed up a PI stem on only one occasion. Both of these guys swore that was their cause for immunity. Each was told later by doctors that it was a dangerous experiment. I don't advise it.
Guy in my church used to eat some at the beginning of every season.
There is online info on how to eat it without touching it (wrap it in a piece of bread) and they advise small amounts of fresh young leaves.
HOWEVER, that being said, many people are (currently) not allergic to it, so it doesn't harm them. Most people are.
The scary thing is that allergies can come and go- maybe years, maybe your whole life, but they can change without you doing anything at all.
You can't really build up immunity, despite all the internet hype about it. DO NOT EAT POISON IVY.
I heard one story of a guy who ate 'too much' and ended up with a bad rash on his bung hole!
A fate worse than death, to be sure.