The Science Behind the Deepest Itch: Poison Oak

Leaves of three ... fuck.

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Aug 8 2015, 11:30am

Image: David Dennis/Flickr

One of my many childhood nervous habits was picking leaves. But not just picking leaves, picking leaves and stuffing them in my pockets, where I would crush them up into tiny balls of finger masticated plant matter. Like any nervous habit, I just did this, with no thinking or planning involved, because it would feel wrong if I didn't. Such are nervous habits. You can already see where this is going.

One summer we were visiting my grandparents in the part of Kentucky that's technically in Illinois, aka Little Egypt or even just southern Illinois. This is where I woke up one morning with a pink blaze across my cheek, like leftover hipster warpaint. Soon enough, the initial rash had become a full-body ordeal, the rare sort of poison oak reaction that becomes an emergency. I couldn't have given myself a worse case of the stuff if I'd tried and, eventually, it was resolved with a shot, presumably a high-dose corticosteroid.

The various varieties of poison oak (and its poison kin) are characterized by their high toxicity to not just human skin, but really any surface of the human body they come into contact with, including airways and lungs. There's a long list of poisonous (to humans) plants, but almost all of them require ingestion to actually be toxic. The toxicodendron genus of plants, which includes poison ivy and oak, is a widespread exception, a frequently encountered nuisance and occasionally a serious threat.

And, at the moment, I am once again covered in its rash. Possibly acquired in any number of places—my home, the Columbia River Gorge, even has its own special poison oak species hybrid—the rash is creeping up the back of my leg (below), spreading up and down my left forearm, and may be on the the move behind my goddamn ear of all places. That it's managed to spread to three distinct regions of my body does not portend good things for the future. Mainly itching, oozing things.

The rash is an allergic reaction and isn't really distinct from any other skin-based allergic reaction, save for its relative severity. The rash is known generally as contact dermatitis, with general symptoms including a hot red, burning, and incredibly itchy rash often featuring raised blisters. The culprit is an oil produced by the toxicodendrons called urushiol. Its mechanisms aren't completely understood, but we have a good outline.

Part of what turns a slight encounter with a poison oak plant into an epic full-body cataclysm is that its action is delayed.

Part of what turns a slight encounter with a poison oak plant into an epic full-body cataclysm is that its action is delayed. The oil can be absorbed by the skin within 10 minutes to the point that it can no longer be washed off, but the ensuing dermatitis reaction can occur up to a full day later. In the interim, the oil, which adheres marvelously to clothing and blankets and most any other surface, has a window to spread all over the place. By the time my rash appeared in Illinois all those years ago, I was basically already condemned to the final outcome. I'd had a full day to ensure its spread.

Urushiol molecules act as haptens, which are a form of immune system trigger. They bind to proteins on the outer membranes of skin cells, changing their structure such that the immune system fails to recognize the cells as legit parts of the body. Immune system scouts known as Langerhans cells detect the now-unrecognized skin cells and travel to the lymph nodes, where T cells are recruited into battle.

So, the immune system wages a potentially very powerful response against what it wrongly perceives as foreign bodies.

The result of this is inflammation, generally. As the immune system has a varied arsenal, this meas a lot of things, but the usual effects when it comes to skin tissue include redness, swelling, heat, and itching. Also: blisters, sometimes oozing and quite nasty. This is the body trying to protect itself from itself. The stuff inside these blisters is not oil from the plant and it does not spread the inflammation.

My policy as an adult has been something like poison oak agnosticism. I'll imagine it doesn't exist and deal with the consequences. For the most part this works out. I get poison oak a couple times a year—usually rock climbing or hiking—and then have a good old time scratching it for a week or so. In the back of my mind, I'm imagining that I'll eventually develop some tolerance and wouldn't that be bad ass?

Image: courtesy of the author

This idea of poison oak/ivy tolerance is, however, hardly settled. For one thing, immune responses to urushiol vary from person to person and vary with age, usually softening as we get older. There are a number of reasonably not bullshitty outlets, from Earthsky to the Straight Dope, that cite non-specific experimental evidence of a developed resistance or tolerance to the oil's effects through repeated ingestion of the stuff in controlled forms. These scraps are, however, dwarfed by the piles of anecdotal evidence, lore, and natural-health-news garbage populating pretty much every imaginable Google search of the topic. As you might expect, a great many of these sources argue that this is a great idea, which it is assuredly not.

Searching PubMed doesn't offer much of anything definitive in favor. One 2005 report speculated that oral exposure to urushiol via mangoes was resulting in tolerance to poison ivy/oak within certain Israeli populations, but this conclusion seems pretty sketchy. A 1983 study found that it was possible to inhibit urushiol sensitization in guinea pigs via injections of pentadecylcatechol, a compound closely related to the oil, directly to their guinea pig hearts. The effect lasted for a single week, after which it dropped off completely.

Promising guinea pig research has since been refuted by human-based trials showing no real benefit tolerance-wise to administering urushiol in pill form, e.g. the sugar pill groups were having the same success rates as the actual urushiol groups. The poison ivy prevention pill remains elusive.

From a 2006 NPR report:

"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."

Immune memory is key as it turns out that urushiol sensitization depends on being exposed to urushiol, e.g. having never encountered the stuff is the best way to not have a reaction to it. The immune system remembers, learning over time that the toxic oil is something requiring a response, even though it's in real-life completely harmless. This is the same difficulty posed by organ transplantation. The new organ is fundamentally harmless, but the immune system (may) attack it anyway, treating it as an outside invader just as it treats those skin cells. "Immune tolerance," Gaspari told NPR, "is the holy grail of immunology.

The upshot is that I'm not helping anything by exposing myself to poison oak and am more likely just training my body to produce increasingly robust attacks against it. The allergic reaction is usually just a nuisance in itself, but it's also not uncommon for repeating scratching to open up tissues to much more dire bacterial infections. As good as it might feel—and it really, really, really does—don't scratch. Or better yet, just don't get it in the first place. And absolutely do not get it in your lungs from breathing smoke from burning poison ivy/oak or in/on your butt from eating it, which is a thing that happens and I can't even fathom how this itch would translate to, uh, there.

As a curious post-script, it's worth noting that plants don't seem to produce urushiol as a defense mechanism. Indeed, birds and foraging animals like deer and raccoons chomp on it with impunity. It's thought that the oil is used by the plants as a wound-sealing or anti-microbial agent. Humans, and some primates, seem to be the only animals with allergies to it at all. Fucking skin.