There are several varieties of toxic-oil plants, poison ivy, poison oak and several others. As I recall, none are actually "ivy" in the classic sense. Most, that I've spotted, grow like low-lying ground cover, or very small bush-like shrubs. Some grows like vines, but those seem to be less common around here.
Toxicity varies, as does the reaction among humans-- some folk are deathly vulnerable, whereas others ignore it completely, not even so much as an itch.
However, again--varies from person to person-- repeated exposure can increase the vulnerability (or just the opposite can also be true).
Me? I was immune as a child, right up until adult hood, for decades, I was the "go to guy" to go pull up the poison ivy, when discovered at Camp. Yes, I'd use gloves, and yes I'd stuff them into plastic trash bags, but I never suffered so much as an itch all those years.
Then? I wasn't immune. I don't know what changed, but now, I get a 2 day rash upon exposure. Hydrocortozone cuts down the reaction, and if I know I was hit, washing with 90% alcohol, then scrubbing with concentrated Dawn dish-washing liquid (using a brush), then alcohol, then re-scrub about 5 cycles? I have no reaction at all, apart from the initial tingling.
If I miss a spot in washing, though, I'm in for a 2 day rash which seems to dry up with treatment.
Oddly enough? My hands seem entirely immune... and I have accidentally transferred exposure from one spot to another, with my fingers-- who act like Typhoid Mary.
I expect it's the thickness of the skin that matters. Perhaps only skin with hairs (and their vulnerable pores) are subject to the effect? Don't know-- not going to experiment, either.
I now am quite certain where the ivy is growing, and I'm very careful along that area-- I'm currently working on a project to get my walk-behind string trimmer up and running again (new-used Kohler engine) and I'll avoid going close in the future.