You’ve likely heard it since childhood: Don’t scratch that bug bite or rash, you’ll make it worse. But why would something that feels so good be bad?
A lot of things can cause itchiness, sometimes serious diseases. Whatever the cause, doctors have long warned that scratching too much can damage the skin. Now researchers better understand why even a mildly annoying itch could put you on an itch-and-scratch cycle if you give in.
How did they find out? In part by putting tiny “cones of shame” onto mice to uncover what happens on a cellular level when an itch gets scratched or left alone.
They also gained insight into why a good scratch, at least at first, brings a sigh of relief. After all, not just people and other mammals scratch, even fish do. The commonality suggests there must be some evolutionary reason, and the mouse experiment hints at a little germ protection — but still not a reason to scratch.
Daniel Kaplan, a University of Pittsburgh dermatologist whose lab studies immune reactions in skin, was exploring a run-of-the-mill type of itch called allergic contact dermatitis, caused by irritants such as poison ivy or nickel in jewelry.
Kaplan’s research team put a rash-inducing irritant on the ears of mice. Normal mice scratched and inflammatory immune cells rushed to the site, increasing swelling. The rash was much milder in mice bred with defective itch-sensing nerve cells. But was the difference really the scratching?
Normal mice put into collars like those veterinary “cones of shame” so they itched but couldn’t scratch gave the answer: They, too, had much less swelling and fewer inflammatory cells.
Kaplan said that evidence matches people’s everyday experiences that scratching really can make things worse.
Ignore a mosquito bite and the itch is “gone in five or 10 minutes for most people,” he said. “But if you start scratching it, it’s your friend for a week,” getting itchier and more inflamed.
To understand what was happening in the skin, Kaplan’s team took a deeper look at mast cells, among the immune system’s first responders. When called into action, they release compounds that can help fight germs or toxins through a compound called histamine, triggering itchy allergic reactions.
Scientists have long known that allergens can activate mast cells. But other signals can summon mast cells, too, including pain. And when we scratch, “we tend to scratch until it starts to hurt,” Kaplan noted.
Pain-sensing 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 do — a double whammy that explains why scratching further inflames itchy rashes or bites.
If we experience pain like touching a hot stove, we’ll learn not to do that again. Yet relief from a good scratch, in evolutionary terms, is positive feedback. Why?
