It just doesn’t seem fair. Why is it animals can trot through the forest without a care, yet we humans can brush up against the wrong plant and break out in a rash?

In addition to being fun to say out loud, urushiol is the active ingredient in poison ivy and poison oak that gives some folks an itchy rash. Notice we said “some” folks. Turns out that not everyone who comes in contact with an offending plant will break out in a rash. About one in five humans will have no reaction whatsoever when exposed to the sticky sap emitted from these plants. The remaining 80 percent will break out in an itchy, blistery rash from contact either direct (from the plant itself) or indirect (say, by touching a pant leg that rubbed against a plant, or from petting a dog that has urushiol in its fur). It all depends upon one’s immune system; most of us have T-cells that recognize urushiol as a foreign substance, or antigen. The body’s white blood cells are then alerted and send out macrophages to “eat” the invader. One side effect, though, is that these macrophages also damage normal tissue, leading to the blistering and itching that is typical to a poison ivy (or oak, or sumac) encounter.

Interestingly enough, most animals happily dine on the leaves of poison ivy with no ill effects. Likewise, your dog may romp happily through forested areas with no effects. Scientists believe that those animals who feast on urushiol-laden plants have built-in immunity to its effect. As for Rover, well, his fur coat keeps the poison ivy sap away from his flesh.