Ever had poison ivy? If so, you probably noticed that scratching made the rash much worse. But scratching an itch, especially one as intense as that caused by poison ivy, is nigh irresistible — and feels so satisfying.
If you think about it, says Daniel Kaplan, who is a dermatologist and immunologist at the University of Pittsburgh, that’s a little curious. When comparing this action to pain, pain causes you to stop what you’re doing and tend to the wound. But the pain also creates a memory that tells you not to repeat whatever you were doing when you got hurt — or to wear gloves the next time you prune the roses.
“From an evolutionary standpoint, there’s a clear and obvious benefit to that pain,” he says. But with scratching, the damage feels good. It’s as if when you cut yourself with a knife, you were immediately compelled to grab the knife and do it again. Where’s the advantage in that?
Because itching and scratching have been observed throughout evolution — dogs scratch, cats scratch, fish scratch — there must be some benefit to it, says Kaplan. Scratching can help remove mites and other irritants from the skin, of course, but that doesn’t explain why we scratch rashes.
Read More: Your Skin Can Become Dry, Cracked, and Damaged in the Winter — Here's Why
Why Scratching Makes the Rash Worse
To solve this enigma, Kaplan and his team began by looking into why scratching makes rashes worse. Their research was published this January in Science.
The answer involves mast cells. Mast cells are activated by allergens (poison ivy, for example), causing inflammation and itching. The researchers showed that scratching activates pain-sensing neurons, causing them to release substance P (a neurotransmitter involved in pain signaling and inflammation), which in turn activates mast cells via a second pathway. So, by activating mast cells through two different pathways, scratching triggers even more inflammation — on top of the inflammation that made you itch in the first place. And that’s why scratching makes the itch worse.
The Benefit of Scratching
So Kaplan and his team discovered how scratching makes the rash worse. But what’s the benefit to all this? So far, it seems as if scratching = bad. Case closed. But not so fast.
Mast cells also play a role in protecting the skin from harmful bacteria. Indeed, the team found that scratching reduces the presence of Staphylococcus aureus on the skin. Staphylococcus aureus is the most common culprit responsible for skin infections.
To Scratch or Not to Scratch?
So, what’s the takeaway here? Should you scratch or not? Kaplan says that’s the question people always ask him. The answer, he says, is simple: “Don’t scratch.” That’s because the bacterial infection is rare; the bigger risk is the allergic reaction that’s causing the itching, he says. Scratching will just make your rash worse and your life more miserable.
Christopher Brooks agrees and adds that scratching breaks down the skin barrier, which can lead to worsening of conditions like eczema and psoriasis. Brooks is an allergist and immunologist at The Ohio State University Wexner Medical Center. However, he stresses that while these findings do help explain why we itch, we should still try to scratch as little as possible. Avoiding scratching won’t make you more likely to get a skin infection.
Kaplan hopes this research, which was supported by the National Institutes of Health and the German Research Foundation, will lead to better therapies for eczema.
“I think it's quite possible the reason [previous] trials failed is they were silencing the wrong pathway on mast cells.” Kaplan adds that his experiments were done on mice (mice who were wearing tiny, adorable Elizabethan ruffs, no less) and have not yet been done on humans.
Read More: Why Scratching an Itch Feels So Good
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
Science. Scratching promotes allergic inflammation and host defense via neurogenic mast cell activation
Avery Hurt is a freelance science journalist. In addition to writing for Discover, she writes regularly for a variety of outlets, both print and online, including National Geographic, Science News Explores, Medscape, and WebMD. She’s the author of Bullet With Your Name on It: What You Will Probably Die From and What You Can Do About It, Clerisy Press 2007, as well as several books for young readers. Avery got her start in journalism while attending university, writing for the school newspaper and editing the student non-fiction magazine. Though she writes about all areas of science, she is particularly interested in neuroscience, the science of consciousness, and AI–interests she developed while earning a degree in philosophy.
