Why do poison oak, poison ivy and poison sumac give me an allergic reaction?

Why do poison oak, poison ivy and poison sumac give me an allergic reaction?

Harold Kaiser (Practicing Allergist and Clinical Professor of Medicine, University of Minnesota Medical School) gives expert video advice on: How are insect allergies treated?; What are "food allergies"?; What are "hives"? and more...

Poison oak, poison ivy and poison sumac contain certain chemicals. They are called urushiols (although they have other names). They induce contact dermatitis, which means that in a fair percentage of the patients who come in contact with it, repeated exposure will cause a reaction. The reaction is a local skin reaction with swelling, itching, redness, pimples, and a very uncomfortable rash. The rash can be spread by scratching the rash and then scratching elsewhere in the non-effected body which means the rash can jump around like a wildfire. We don't know why these chemicals are so inducing; all we know is that in the same way that certain medications like penicillin cause more allergies than other medications, these plants are more likely than others to induce a contact dermatitis. The other interesting thing is the contact dermatitis typically occurs hours to a couple of days after exposure. So you can be out in the woods and then on the way home, and then an hour or two later you start to develop the itching and the rash, and then you get the typical follicles and the redness of the poison oak, poison ivy, or poison sumac.