What is poisonous in poison ivy?
Justin M. Nolan, assistant professor of anthropology in the J. William Fulbright College of Arts and Sciences, replies:
For about 85 percent of all human beings, exposure to poison ivy, Toxicodendron radicus, results in almost immediate contact dermatitis, or an itchy, painful rash that sometimes produces blisters when reactions are especially severe. While considerable variation in sensitivity to poison ivy exists among humans, this rash itself is caused by the organic oil urushiol, a powerful toxin found throughout all parts of the poison ivy plant: the leaves, stems, roots and fruits.
Identifying the plant is easy enough: remember the old saying, “leaves of three, leave them be.” That is, the leaves occur in a trifoliate pattern, while the woody plant can sprawl along the forest floor, climb upwards as a vine, or occasionally stand alone. Deer, horses and other animal browsers freely consume the sap-rich leaves of poison ivy without consequence, and birds consume the fruits and disperse its seeds accordingly. But for humans, even the slightest exposure to the compound can trigger a rapid, alarmingly painful allergic response. This happens because our immune systems recognize urushiol as a foreign invader and send T-cells to the affected region within minutes of contact.
While washing with warm water is the best method to alleviate the allergic symptoms, the most widely known and effective folk treatment of poison ivy I have documented in the Ozarks is jewelweed, Impatiens capensis. Jewelweed is a common annual flowering herb found locally, frequently in damp soils, near running water. The leaves of jewelweed are opalescent and sparkle like a jewel under water, and thus the plant can be identified rather easily. When jewelweed is crushed and applied directly to irritated skin, most sufferers report rapid relief and reduction of itchiness and inflammation.
Evolutionarily, the urushiol found in poison ivy is interesting because most bioactive oils and secondary alkaloids occur throughout the plant kingdom as a by-product of adaptation and selective pressure. In other words, compounds like urushiol evolve initially by accident, but through time, they perhaps enhance the survival and propagation of a given species by offering a selective advantage against herbivores. While the sap of poison ivy – the main vehicle for urushiol – plays a role in the plant’s healing process when its tissues are damaged, urushiol forestalls fungal and bacterial growth. To summarize, what’s good for poison ivy usually spells serious distress for the outdoors enthusiast!