Well then you all better get immuned to the stuff
It's a common misconception that the chemical in poison ivy is part of the plant's defensive system. Also humans seem to think this is an evil plant, but to use a term from Dogwood, that's just flawed human-centric thought.
Actually, it's a very good food source for many animals and it's really only the humans (some humans) that don't respond well to it; it probably has to do with us being invasive species or something 
Here's an interesting link, I've also read that the "active" ingredient is believed to be used by the plant simply for water retention, but it's an open question...
Excerpt:
"So what's going on here? Why is it that humans react to poison ivy in ways that other animals do not?
One explanation is that poison ivy produces urushiol. Urushiol is a mixture of phenolic compounds and is the substance that is responsible for our reaction to the plant. The production of urushiol by the plant though is thought to provide a protective benefit to the plant: they are less likely to get eaten and are therefore more likely to survive to reproduce than their competitors.
But since grazing animals like deer have no ill effects from feasting on poison ivy, this explanation doesn't make much sense. Maybe the urushiol gives some other benefit to the plant itself or maybe just has no benefit to the plant at all. The evolution of the poison in poison ivy remains an open question.
The interesting thing though is that technically, urushiol is not a poison at all! It does no damage of its own to tissues. In fact, the red, painful, oozing rash that can result from exposure to urushiol is merely the result of an allergic reaction. It is our OWN bodies that are doing that to us, not the plant."
Poison Ivy Getting Stonger
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Originally Posted by Dogwood
