Should You Scratch That Itch? Science Has a Surprising Answer

Despite the saying “happiness is having a scratch for every itch,” itching often makes the itchy skin condition worse. So why is the instinct so strong?

In a study with mice, researchers from the University of Pittsburgh uncovered why scratching worsens inflammation and swelling—and why humans and other animals may have, despite this, evolved to scratch, fueling the so-called “itch-scratch cycle.” According to their results, published today in Science, scratching an itch appears to trigger antibacterial inflammation—a surprising finding with potentially important implications for treating inflammatory skin conditions.

“At first, these findings seemed to introduce a paradox: If scratching an itch is bad for us, why does it feel so good?” Daniel Kaplan of the University of Pittsburgh, senior author of the study, said in a university statement. “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.”

Led by Andrew Liu of the University of Pittsburgh, the researchers gave mice symptoms of allergic contact dermatitis, an eczema in which allergens or skin irritants cause itchy rashes—and scratching those rashes aggravates the situation. They induced the symptoms in normal mice, as well as mice without itch-sensing neurons.

When normal mice scratched their ears, the appendages swelled and filled with neutrophils (inflammatory immune cells). Some normal mice, however, wore collars similar to a dog’s “cone of shame” to prevent them from scratching, and their ears—along with those of mice without itch-sensing neurons—had milder inflammation and swelling. This difference in outcomes confirms that, unfortunately, your parents were right: scratching does make it worse.

The researchers also demonstrated that scratching prompts pain-sensing neurons to release a chemical known as substance P, which triggers mast cells: cells involved in the immune system, causing inflammation and itchiness via neutrophils.

“In contact dermatitis, mast cells are directly activated by allergens, which drives minor inflammation and itchiness,” Kaplan explained. “In response to scratching, the release of substance P activates mast cells through a second pathway, so the reason that scratching triggers more inflammation in the skin is because mast cells have been synergistically [collectively] activated through two pathways.”

Mast cells are behind various unwelcome skin conditions, but they’re involved in protecting us from pathogens, as well. In fact, the researchers found that scratching also led to a decrease in Staphylococcus aureus (the primary bacteria responsible for skin infections) on the skin.

“The finding that scratching improves defense against Staphylococcus aureus suggests that it could be beneficial in some contexts,” said Kaplan. “But the damage that scratching does to the skin probably outweighs this benefit when itching is chronic.”

By highlighting the role of mast cells within the itch-scratch cycle, the study could have a strong impact on future therapies for inflammatory skin conditions. It also reminds us that nothing is worse than having an itch you can never scratch…

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