Why Scratching an Itch Makes It Worse — And Why It Feels So Good
WASHINGTON (AP) — You’ve probably heard it since you were a kid: Don’t scratch that insect bite or rash or it will get worse. But why should something that feels so good be bad?
Many things can cause itching, sometimes serious illness. Whatever the cause, doctors have long warned that scratching too hard can damage the skin. Now researchers better understand why even a mildly annoying itch can send you into an itch-scratch cycle if you give in.
The Mouse Experiment That Changed Everything
Among other things, researchers put tiny “cones of shame” — the same type of veterinary collars used on dogs and cats after surgery — on mice to find out what happens at the cellular level when an itch is scratched or left alone.
Dr. Daniel Kaplan, a dermatologist at the University of Pittsburgh whose lab studies immune responses in the skin, studied a common type of itching called allergic contact dermatitis, which is caused by irritants like poison ivy or nickel in jewelry.
Kaplan’s research team applied a rash-inducing irritant to the ears of mice. Normal mice scratched themselves and inflammatory immune cells flocked to the site, increasing the swelling. The rash was much milder in mice bred with defective itch-sensitive neurons. But was the difference really in the scratching?
Normal mice placed in collars so they could itch but not scratch themselves gave the answer: They, too, had much less swelling and fewer inflammatory cells.
“The evidence is consistent with people’s everyday experiences that scratching can really make things worse,” Kaplan said.
The Itch-Scratch Cycle Explained
Ignore a mosquito bite and the itching is “gone in five or 10 minutes for most people,” Kaplan said. “But if you start scratching, it’s your friend for a week.” The itching and inflammation increase.
To understand what’s happening in the skin, Kaplan’s team took a closer look at mast cells, which are among the immune system’s first responders. When they spring into action, they release compounds that can help fight germs or toxins — or trigger itchy allergic reactions via a compound called histamine.
Scientists have long known that allergens can activate mast cells. But other signals can also be triggered by mast cells, including pain. And when we scratch, “we tend to scratch until it starts to hurt,” Kaplan noted.
Pain-sensitive nerve cells release a chemical messenger called substance P. In findings published last year, Kaplan’s team reported that substance P can activate mast cells through a different molecular pathway than allergens — a one-two punch that explains why scratching further inflames itchy rashes or bites.
Why Does Scratching Feel So Good?
When we feel pain, like touching a hot stove, we learn not to do that again. But from an evolutionary perspective, relief is fundamentally positive feedback. Why?
A long-held theory is that it may help living creatures repel parasites such as fleas or mites. But Kaplan was also intrigued by other labs’ findings that mast cells can fight off a common type of skin bacteria called Staphylococcus aureus. So his team infected mice and then repeated the itch experiment with the cone of shame. In fact, those who scratched had fewer germs on their ears, possibly due to the added inflammation or another connection related to mast cells.
But that’s not enough to change health recommendations.
“Ultimately, scratching is harmful,” Kaplan emphasized. “You should avoid scratches,” he said, but acknowledged that it was “easier said than done.”
How to Treat a Mild Itch
What combats an itch depends on its cause, and there is a need for better treatments. Currently, antihistamines and certain other hives medications can relieve itching caused by mast cells. Drug companies are experimenting with other approaches, called MRGPRX2 blockers, that target the signaling pathway that Kaplan’s team links to scratching. Kaplan hopes that a better understanding of this signaling pathway could ultimately help with skin diseases such as chronic eczema.
To combat summer itching from insect bites, poison ivy, and other types of contact dermatitis, dermatologists recommend anti-itch balms such as hydrocortisone cream, calamine lotion, or oatmeal baths.
Another trick from Kaplan: Menthol-containing creams can temporarily trick the skin into feeling cold instead of itching, for just long enough that “if you don’t scratch, you break the itch-scratch cycle,” he said. “It’s like a cheat code.”
