Pill To Prevent Poison Ivy Itch Proves Elusive

If you haven't had an allergic reaction to poison ivy yet, just wait. You're never too old. Here's what you need to know to minimize the nastiness of the encounter.

Poison ivy i i

More than 85 percent of people will develop an allergic reaction if exposed to enough of the sap of poison ivy, experts say. iStockphoto.com hide caption

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Poison ivy

More than 85 percent of people will develop an allergic reaction if exposed to enough of the sap of poison ivy, experts say.

iStockphoto.com

If you've escaped the itchy rash of poison ivy and poison oak so far, don't gloat; it's probably only a matter of time. Experts say more than 85 percent of people will develop an allergic reaction if exposed to enough of the sap of these plants, at least one of which is found in every state in the U.S. except Alaska and Hawaii. And those allergic reactions can make you miserable.

"I see a lot of different kinds of rashes," says Dr. Jim Marks, who heads Penn State University's dermatology department in Hershey, Pa. "And almost everyone I run into tells me, 'The worst rash I ever got was to poison ivy.'"

Over the last hundred years, generations of skin doctors and immunologists have worked hard to develop some sort of pill that will reverse this sensitivity to urushiol — the sticky resin in poison oak, ivy and sumac that triggers the allergy. Back in the 1980s, Marks thought he was close.

"There were a number of animal studies suggesting that if you somehow could avoid the skin — say by injections or orally — you could induce tolerance to the resin," he says. A few anecdotal stories from people who've actually tried nibbling on the leaves suggested the same thing. But don't try this at home! Others who've eaten the leaves just ended up with a horrible rash — from stem to stern.

For Marks, the most intriguing bit of evidence came not from a lab or the woods, but from a factory outside of Philadelphia — at a place that grinds up the shells of cashew nuts to make friction dust for the brake linings of cars. Turns out those cashew shells are loaded with urushiol, the same stuff that makes poison oak and ivy such a menace. The workers started itching. As you might expect, Marks says, they either got over the rash in a month or so, or they had to leave the factory.

But after that first month of itching, the workers who stayed were no longer allergic to the cashew shells. What's more, Marks says, after working at the factory, some who had a history of getting poison ivy blisters — some of them severely — "could roll around in poison ivy and not have problems."

Testing Tolerance

Marks figured that maybe constantly ingesting some of the particles of urushiol in the factory air had somehow helped the workers develop a tolerance to it. That gave him hope that a toned-down version of urushiol in pill form might also induce tolerance.

So he and his team got to it. Chemists were indeed able to come up with a safe pill that worked just the way they'd hoped — in guinea pigs. At first, it seemed promising in people, too, when the researchers tested it against a sugar pill, or placebo. In the midst of the study, even some of the volunteers were convinced the pills they were getting had cured them.

"This one subject who had always gotten bad poison ivy every summer was sure that he had gotten the active [pill]," Marks says, "because he didn't have poison ivy at all."

Unfortunately that wasn't the case. "When the study was over and we checked to see who'd gotten what, we learned that indeed, he'd been on the placebo."

'Immune Memory'

It's been 20 years since Marks' study with the workers in the cashew factory. Why aren't scientists any closer to that anti-poison ivy pill?

"It's very complicated," says Dr. Anthony Gaspari, who chairs the Department of Dermatology at the University of Maryland School of Medicine in Baltimore. "I tell patients that the skin is armed and dangerous."

The skin has immune system soldiers, Gaspari explains — white blood cells lined up and ready for attack against any harmful germ or chemical that might try to break through. That's great for battling bacteria in a cut or a scrape, but in the case of poison ivy, it does more harm than good.

"There's something about urushiol," says Gaspari, "that tricks the immune system into thinking, 'Boy, this is something really dangerous. And we have to remember this, and the next time we see it, we're going to attack it." That "immune memory" is remarkably stubborn, he says. "It can't be turned off."

Gaspari says that if scientists could figure out how to get those immune system soldiers to step down when they're not needed, they'd be closer not only to preventing the rash of poison ivy, but also to solving the heartbreak of rejection in organ transplantation. They'd be closer to curing other illnesses like lupus and multiple sclerosis that are linked to glitches in an overactive immune system.

"Immune tolerance," he says, "is the holy grail of immunology."

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