|This is my own personal experience.
I never got Poison Ivy when I was young, and I touched it many times. By the time I was 50 I got it, and that made me very sensitive, so I get it every year for about 10 years now, multiple times a year.
The first time I get it in the season its the worst, and it gets progressively milder each subsequent time I get it thru the season.
I guess I am building up immunity.
Then over the winter, when I am not exposed, I lose immunity and the first time I get it the following year it is as bad again as the first time in each of the preceding years.
This leads me to believe that the body can build up immunity, after repeated exposure, but the immunity does not last.