Today's useless fact - Why is poison ivy poisonous?

Poison ivy, poison oak, and poison sumac are all members of the
cashew family and cause contact dermatitis, or skin rash. Poison
ivy produces an alcohol (catechol) that is stored within its
plant tissue. Merely touching the leaves will cause no reaction.
The leaves must be broken to release the allergin.

These plants evolved poisons to protect themselves from plant-
eating animals, not just to ruin our camping trips.
If you wash your hands and other areas of the body within ten
minutes of exposure, you can avoid an allergic reaction, as the
allergin cannot penetrate the skin this quickly. However,
wash with water alone, because soap removes protective oils from
the skin.

In the United States, poison ivy and western poison oak are
responsible for more clinical cases of dermatitis than all other
plant and chemical sensitizers combined. Believe it or not, in
California over 50 percent of all workmen's compensation
cases are caused by poison oak-induced dermatitis. That's one
itchy state!

~source used: "Thoughts for the Throne" by D. Voorhees