Why it feels good to scratch that itch: the immune benefits of scratching

Scratching where it itches reduces the presence of potentially harmful bacteria on the skin, studies in mice show

Itchy-scratchy: experiments show that there are benefits to scratching ― although it can also aggravate skin disease. Credit: Getty

Scratching a mosquito bite can offer a moment of bliss, and now scientists have learnt why: scratching activates an immune response that helps to protect the skin against harmful infections, at least in mice.

The findings could also explain why humans find a good scratch satisfying. The study¹ was published today in Science.

“The exciting thing about this study is that there's now a molecular basis” for how scratching causes inflammation, says immunologist Aaron Ver Heul at Washington University School of Medicine in St. Louis, in Missouri, who was not involved in the study. The work, he added, is “rigorous” and “really well done”.

Why we scratch

Almost all animals scratch, even though scratching too much can damage the skin. The classic explanation for scratching is that it removes parasites and irritants. “But we’ve always thought there might be other reasons,” says Ver Heul. After all, some parasites, such as mosquitoes, are long gone by the time itching starts.

To get to the root of scratching, skin immunologist Dan Kaplan at the University of Pittsburgh in Pennsylvania and his colleagues painted a synthetic allergen on the ears of mice. This induced a form of skin inflammation called allergic contact dermatitis, which is caused by contact with an allergen such as the oil in poison ivy. When control mice scratched, their ears swelled up and became full of neutrophils, a type of immune cell.

But mice that wore tiny Elizabethan collars — the rodent version of the ‘cone of shame’ that dogs wear — couldn’t scratch their irritated ears. These animals’ ears sported less swelling and fewer neutrophils than the ears of control mice. Bioengineered mice that were lacking an itch-sensing neuron showed a similarly muted reaction. This experiment showed that scratching itself worsens inflammation.

From itch to scratch

To learn more about what happens after scratching, the scientists studied ordinary mice that were allowed to scratch their itchy ears. They noticed that at scratched sites, pain-sensing neurons released a potent nervous-system messenger called substance P.

The substance P turned on the activity of key white blood cells called mast cells, which are central to triggering allergy symptoms. The mast cells recruited neutrophils to the scratched site, driving inflammation.

It was previously known that mast cells can be turned on directly by allergens. The authors’ work revealed that mast cells can also be turned on indirectly, by scratching and the sequence of steps it causes.

Inflammation by neutrophils was greatly increased if the mice scratched and activated both pathways. Scratching was a key component in the development of inflammation. “That was a bit of a surprise,” says Kaplan.