While urushiol makes the plant very unpleasant for us, it is all but useless as a defense, since it usually takes quite a few hours before any discomfort shows up. Instead, it causes the sap to form a hard lacquer that seals up any injury to the plant.

The itching rash is actually an allergic reaction to the Urushiol in the Poison Ivy plant. Urushiol is actually a group of chemicals, that vary somewhat in their composition. Some people are not allergic to them at all, some people are allergic to most of them, and some people are only allergic to some of them. As with other allergies, this can change as you age, so just because you never reacted to Poison Ivy as a child does not mean you are still ivy-proof now.