My comment in this thread might address this.

Like most allergens, people have very different reactions to it and can grow tolerance over time with exposure.

Seems like it might vary just as much as poison ivy exposure, and for the same reason (or nearly so, since the plants may also have other irritating chemicals that aren't the same between species). Some people are totally immune to poison ivy and don't get any reaction from physical contact with it, although not too many.