Is there a vaccine for poison ivy?

Nope, sorry. And you must have been one of the lucky ones even as a kid.

In decades past, doctors did try exactly what you remember – giving small but increasing oral doses of the allergy-causing substance from poison ivy in hopes of “desensitizing” people, that is, making their immune systems more “tolerant” or less reactive, to poison ivy. But these attempts usually failed, and sometimes made people worse, said Dr. Robert Stern, chief of dermatology at Beth Israel Deaconess Medical Center.

If doctors gave the substance orally, it passed through the digestive system and caused a red, itchy rash in a particularly unpleasant place, the rectum. If they tried to desensitize people topically, that is, by rubbing a tiny bit of poison ivy on the skin, that elicited exactly the response they were trying to avoid- a red, itchy skin rash.

An allergic reaction to poison ivy is different from allergic reactions to things like pollen. Depending on your genetic makeup, pollen from a particular kind of tree, say a birch, may cause an immune reaction – release of a type of antibody called IgE. This antibody sits on the surface of mast cells, immune cells that line the nose, throat, gastrointestinal tract and skin. The next time pollen enter the system, it combines with these antibodies, causing the mast cells to release histamine and other chemicals that make blood vessels dilate and leak proteins into tissues, causing swelling – the stuffy noses of allergy suffers.

With poison ivy, the exaggerated immune response is not triggered by antibodies, but by cells called T cells. The first exposure to poison ivy won’t do much, but the second time around, the T cells are primed to pump out chemicals -cytokines and interleukins- that trigger inflammation.

Even if there were a good way to desensitize a person to poison ivy, it might be dangerous, said Dr. Mariana Castells, co-director of the allergy and clinical immunology training program at Brigham and Women’s Hospital. T cells trained to ignore the nastiness of poison ivy might also ignore viruses and bacteria. “If you tolerized people” to the substance in poison ivy, “you might tolerize people to things they should be reacting to,” she said.

Bottom line? Stay away from poison ivy if you can (and don’t burn it if you’re cleaning up your garden!) If you do get a poison ivy rash, wash it gently with water (no soap), take oral antihistamines like Benadryl to help with the itching or anti-itch lotions and topical steroids like Clobetasol. If that fails, your doctor may prescribe oral steroids like Prednisone.