You’ve been told not to scratch that itch—but why does it feel so good? Researchers studying the science behind scratching found that while it worsens inflammation and swelling, it also has certain benefits which may explain why the natural urge feels so irresistible.
In a recent study published in the journal Science, researchers examined how itching affected the skin of mice with allergic contact dermatitis, a type of eczema.
“Scratching is often pleasurable, which suggests that, in order to have evolved, this behavior must provide some kind of benefit. Our study helps resolve this paradox by providing evidence that scratching also provides defense against bacterial skin infections,” said senior author Daniel Kaplan in a news release.
For the trial, the researchers used itch-inducing allergens to induce symptoms of eczema in the ears of normal mice. While some mice were allowed to scratch, others were restricted from scratching using tiny collars, similar to those used by dogs.
The results were striking: mice that scratched developed swollen, inflamed skin packed with immune cells called neutrophils, while those unable to scratch had much milder inflammation. This confirmed that scratching worsens skin irritation rather than relieving it.
The researchers explain that this is because scratching an itch sets off a chain reaction in the skin. Pain-sensing nerves release a chemical called substance P, which activates mast cells or the immune cells that regulate inflammation and itching. Normally, mast cells respond to allergens, causing mild itchiness and swelling. But scratching triggers a second wave of activation through substance P, intensifying inflammation and making the itch even worse.
However, mast cells not only cause irritation, they also help defend against bacteria and other germs. This made researchers curious to find out if scratching actually influences the skin’s microbiome.
In further experiments, the team showed that scratching reduced the amount of Staphylococcus aureus, the most common bacteria involved in skin infections, on the skin. “The finding that scratching improves defense against Staphylococcus aureus suggests that it could be beneficial in some contexts. But the damage that scratching does to the skin probably outweighs this benefit when itching is chronic,” Kaplan said.
