Scratching an itch has both good effects and bad

Don’t scratch that mosquito bite! Chances are you’ve heard this advice before — and had a hard time following it. So you may already know that scratching can feel good at the time, but may not help in the long run. A study in mice finds that scratching can have both positive and negative effects.

Researchers shared these findings January 31 in Science.

First, the bad news. Scratching itchy ears triggered inflammation. That’s a local immune response that causes redness — and more itching.

An itch can begin when a substance (such as an oil in poison ivy) activates mast cells. These immune cells release itch signals and kick off inflammation. Scratching boosts that process, the new study suggests.

“The act of scratching is actually triggering the inflammation by [working] with mast cells to make them more effective,” says Daniel Kaplan. He’s a dermatologist and immunologist at the University of Pittsburgh in Pennsylvania. He also coauthored the new study.

Mice that couldn’t scratch their itchy ears (thanks to tiny cones on their heads) had less inflammation than mice that scratched. The same was true in mice that didn’t feel the itch.

Kaplan relates the results to a mosquito bite. “Most of the time, it’ll go away in five, 10 minutes,” he says. “But if you start scratching it … you get a really big, inflamed, itchy lesion on your skin that can stick around for several days. It’s a lot worse. And I think this could be a mechanism that explains why.”

Now onto the good news. Scratching lessened the amount of potentially harmful bacteria (Staphylococcus aureus) found on the skin of itchy mice. That may be because of a heightened immune reaction that scratching can prompt.

“Scratching can have a benefit in the context of an acute infection,” Kaplan concludes. But too much scratching can rip the skin and usher in more bacteria, he cautions. “In that sense, scratching, through a different mechanism, also makes things even worse.”

In recent years, scientists have uncovered lots of new details about itching. And they’ve developed new ways to fight harmful forms of it, Kaplan says. The new study may point out other new approaches to treating itch.

So is scratching good or bad? “It’s both!” Kaplan says.