[paul.fr]

Just how is poison ivy poisonous? Oh, and what are the symptoms?

[kdlynn]

at least some of the symptoms are itchy red bumpiness

[another_someone]

at least some of the symptoms are itchy red bumpiness

From what I can ascertain, that is probably the dominant symptom for most people.

http://en.wikipedia.org/wiki/Poison_ivy#Effects_on_the_body

The reaction caused by poison ivy, urushiol-induced contact dermatitis, is an allergic reaction. Around 15% to 30% of people have no allergic response, but most if not all will become sensitized over time with repeated or more concentrated exposure to urushiol.

For those who are affected by urushiol, it causes a very irritating rash. In extreme cases, corticosteroids can be needed to treat rashes and severe itching. The first symptom of contact is a severe itching of the skin that develops into reddish colored inflammation or non-colored bumps, and then blistering of the skin occurs. In severe cases, clear fluids ooze from open blistered sores. Once the urushiol poison has had contact with the skin, it is quickly bound to the skin. The poison can be washed off within a short time after contact, but once bound to the skin, it cannot be washed away.
People who are sensitive to poison-ivy can also experience a similar rash from mangoes; the skin of mangoes has a chemical compound similar to urushiol.