When I occasionally battle an infestation of poison ivy, I wonder about its place in the ecosystem. One might assume that it developed its "poison," called urushiol, to protect itself from being eaten or otherwise disturbed. It turns out that most animals are not at all allergic to poison ivy, and will happily eat it! Humans, on the other hand, can experience long-lasting misery from just touching the plant. Why do our bodies react so violently to something that's not even harmful? MinuteEarth goes into our immune system to explain how this unique chemical sends us into overdrive. The video is barely more than two minutes long; the rest is an ad.