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It sure seems like some plants really don't want you hiking through their woods, but maybe your immune system is just overreacting.

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Nothing can ruin your vacation quite like an encounter with one of nature's purveys of itchiness.  Of course I'm talking about poison oak, poison ivy, poison sumac, and their notorious relatives.  Yes, these botanical nightmares are clearly out to make camping and hiking as miserable as possible, but maybe they're not?  Maybe they were just hanging out in the forest enjoying the fresh air and trying to keep harmful microbes at bay until you and your overeager immune system happened to come along.

Poison ivy and the like are innocent.  These plants all produce an oily resin called urushiol, and it isn't just in their leaves, it's in every part of them, and it remains even after the plant has died.  This urushiol is what causes that notorious itchy rash you get or what doctors call urushiol-induced contact dermatitis.  Now, the leaves have to be crushed or somehow damaged in order for the urushiol to actually contact your skin, so you won't get it just by touching the plant. 

Of course, leaves and such are easily damaged by insects, passing animals, or a stiff breeze, so just because you didn't damage the leaves yourself doesn't mean something else didn't get there first.  In fact, because urushiol causes such a violent rash in people, there's this pervasive idea that the resin evolved as a defense mechanism against large mammals like us, but that's not true, or at least if it was a defense mechanism, it would be a really lousy one, since urushiol doesn't bother most animals that encounter it, just us and apparently hamsters, but it has no effect on the animals that actually feed on the plant, like deer, insects, and birds.  

That's why scientists think it's more likely that urushiol evolved as an antimicrobial.  It's quite effective against much smaller plant pests.  There's even been some speculation that birds that eat the seeds actually benefit from urushiol's antimicrobial and anti-parasitic properties.  It's simply an unfortunate accident of evolution that makes these plants incompatible with humans, so the rash, oozing blisters, and relentless itching, it's all pretty much down to cosmic unfairness.

Urushiol interacts with your skin cells, specifically ones that express a protein called CD1a.  CD1 proteins help the body spot invaders and sick cells.  They bind to specific fats, then show those fats to the body's immune cells, except human CD1a has the unfortunate tendency to set off attacks in response to things that aren't pathogenic and it just so happens that urushiol is one of those things.

Urushiol-loaded cells activate the body's t cells, those vigilant warriors of the human immune system.  Then the t-cells release two proteins called interleukin 17 and interleuken 22 and they're what make you itch.  The redness, swelling, and blisters all occur because your immune system harms your skin cells in its attempts to eradicate a bit of harmless oil. 

Now, of course, this doesn't really apply to those 10-15% of humans who don't seem to be affected by urushiol at all.  For some reason, nature has seen fit to spare some people from the horrors of urushiol, but there hasn't been a lot of scientific research into why that is.  It might be because urushiol-induced contact dermatitis is basically an allergy and allergies occur when the body mistakes a harmless substance for something harmful, and like with allergies, some peoples' immune systems just go haywire when encountering urushiol while others' don't. 

It's just that most people are allergic to urushiol, but also some of the people who say they're immune probably aren't really immune.  Most people don't react to urushiol the first time they're exposed, so a person might believe they're immune when in fact, they could go on to develop an allergic reaction if they touch it again, and even if they've had a couple of exposures with no reaction, they could still develop a sensitivity later in life and increased exposure is thought to increase the likelihood of developing sensitivity, so if they roll around in a bunch of poison ivy to show off to their hyper-allergic friends, might end up experiencing some itchy, itchy karma, though weirdly enough, sometimes sensitivity to urushiol fades as people get older.

Scientists aren't really sure why, but it might have something to do with the fact that your immune system weakens as you get older and a weaker immune system may not mount as strong a defense against urushiol, so that's one advantage to getting older.  At any rate, whether you think you're immune or not, or may have become less sensitive over time, you probably don't want to push your luck.  Urushiol-induced contact dermatitis is miserable at any age, so it's always a good idea to avoid whichever version of poison whatever is in your neighborhood.

If you want to learn more about some of the quirks of the human immune system, you might like our episode on how having parasites could actually be good for you, and of course, before you go, be sure to click that subscribe button and ring that notification bell.  Ding ding ding ding ding ding ding!