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Avoiding Rash Decisions
Illustration by Adelaide Tyrol
“Plants are in the business of making themselves inedible,” says Tom Vogelmann, a plant biologist at the University of Vermont. Being unable to run and hide from predators, they’ve spent millions of years figuring out other ways to fight back. Some have developed armor – tree bark, for instance, or thorns. Others employ more advanced chemical defense systems. These chemical mechanisms range from shock and awe (an herbivore who eats an amanita-type mushroom could drop dead) to on-the-sly (some wild yam species contain a contraceptive molecule that works to render a predator sterile – a brilliant bit of lose-the-battle, win-the-war strategy).
Walk through a poison ivy patch and you’ll learn firsthand about plant defense mechanisms. The resulting rash you’ll get, the annoying pain you’ll feel, isn’t personal – the toxins in these plants weren’t meant to target humans specifically. Still, the fact that we’re merely collateral damage in a chemical war that has been raging for eons is of little consolation to someone soaking in a bathtub full of calamine lotion.
Poison ivy (Toxicodendron radicans) can be identified by its almond-shaped leaves that have subtle teeth along the edge. The leaflets appear in clusters of three, and the middle leaflet has a longer petiole. The plant grows both as a ground cover and as a woody vine that climbs trees by means of aerial roots.
If you brush up against any part of a poison ivy plant (or come in contact with a pet or a garden tool that has brushed up against poison ivy), you might contract a rash. Rashes typically appear a day or two after you’ve touched the plant and are characterized by red swelling and small blisters. The intensity of the rash varies among individuals.
To understand why some people seem immune to the plant while others get a rash just by looking at it, one must first understand the science behind the body’s reaction. Poison ivy contains a chemical compound called urushiol. When you touch the plant, the urushiol is transferred to your body and absorbed through your skin, where it binds harmlessly to your skin cells. Left to their own devices, your skin cells would simply metabolize the compound, and you’d never even know you’d touched the plant. But if the body’s immune system gets wind of things, all hell breaks loose.
Inside your body, your T-lymphocyte cells are constantly roaming around looking for invaders (high-school science teachers often call them “cop” cells). If there are no T-cells in the vicinity, the urushiol compound will not be discovered and no rash will ever develop. But if the chemical is spotted, your T-cells call for military backup of killer T-cells that swarm in, latch on, and release toxic enzymes (this is why it sometimes takes several days before the rash appears – it takes time for the cells to multiply and for reinforcements to appear). The enzymes destroy both the invading compound and your healthy skin cells. The effect is red blistering and an itchy rash.
Now some people’s T-cells, for whatever reason, don’t recognize urushiol. These lucky folks can roll in a patch of poison ivy with immunity…and impunity. If you’ve brushed up against the plant, though, with no ill effect, don’t be so sure you’re one of the lucky ones. Touching poison ivy is like playing Russian roulette – the more contact you have, the greater the chance of its “discovery” in your body. And once you’ve been sensitized, every time you touch the plant, the reaction will happen more quickly. This is why gardeners are often hypersensitive to poison ivy: they’ve been over-exposed, and their T-cells are just waiting to pick a fight.
All parts of the plant can give you a rash. If you burn poison ivy, urushiol can attach to smoke particles, which can land on your skin and infect you; worse, you can inhale the toxin, and it can severely damage your lungs. Washing does remove the oil, but it can also spread it. Air alone can not spread the compound. The liquid in blisters is harmless and will not spread the rash, but try not to scratch, as the bacteria in your fingernails may cause additional skin infections. Folk wisdom suggests that jewelweed sap will ease the rash, but at least two controlled clinical studies have shown that jewelweed is no more effective than a placebo in poison ivy treatment.
While the toxins in poison ivy are remarkably effective in keeping humans away (Leaves of three, let it be! Hairy vine, no friend of mine!), it doesn’t work as well on avian predators. The many species of birds that are fond of the berries are culprits in spreading the plant as they deposit seeds along their autumn flyways.
Dave Mance III is the editor of Northern Woodlands magazine.
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