It all boils down to the skin's reaction to a nasty little oil called urushiol (ooh-roo-she-all). Urushiol is found in all parts of the poison ivy, oak and sumac plants. When urushiol gets on the skin it binds to the skin within 10-20 minutes. At that point the urushiol becomes extremely difficult, if not impossible, to get off the skin with soap and water.
The rash from poison ivy, oak and sumac is the body's natural reaction to the allergen. Contact with this annoying oil produces a rash in three out of four people. The rash can begin within a few hours after contact, or it can start three to five days later.