It is difficult to resist scratching a itch – and the evolution can be blamed

It is difficult to resist scratching a itching – and there could be an evolutionary reason for this, suggests a new study.

People are generally advised not to scrape the skin of the skin – whether due to an insect bite or a chronic condition – because too much abrasion can increase the risk of infection by injuring the skin and can worsen itching. However, a study published Thursday, January 30, in the journal Science have found that a certain amount of scratching can be useful.

It turns out that scratching a itching increases inflammation And stimulates the body’s immune response on a itching injury site, helping to ward off the infection, the researchers discovered. The results can help explain why the scratches have been preserved on several species.

“If scratching a itching is bad for us, why does it feel good?” Co-author of the study Dr Daniel KaplanA dermatologist and immunologist at the University of Pittsburgh, said in a statement. “The scratches are often pleasant, which suggests that, for having evolved, this behavior must provide a kind of advantage.”

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To discover what this advantage could be, Kaplan and his colleagues have raised mice which had certain itching neurons in their deactivated bodies. The researchers then dabbed an allergen on the ear of each mouse to trigger a feeling of itching. They compared the allergic reactions in mice modified to reactions in normal mice, some of which wore necklaces which prevented them from scratching, similar to the “shame cone” that a dog could wear after a trip to the veterinarian.

The normal mice without cones scraped their ears in response to the allergen and have developed swelling. Researchers have found that this inflammation results from pain detection neurons releasing a substance that activates “mastocytes”, which act as alarms in the immune system. To trigger an inflammation, these mastocytes invoke another type of immune cell called neutrophil which helps correct germs behind infections, among other roles.

Meanwhile, mice with cones, as well as those that could not feel itching, have developed much less swelling in their ears. This suggests that scratches were responsible for most of this inflammatory response.

Mastocytes invoke body defenses against infection, so scratches can protect against infection by activating these cells, have proposed the researchers.

To test their hypothesis, the researchers exposed the ears of each mouse to a substance inducing itching followed by Staphylococcus aureusA bacterial species which generally causes skin infections. After a day, normal mice without cones had 10 times less S. Aureus Bacteria on their skin, compared to mice with cones and those without detection detection of itching. This suggested that scratch has strengthened mouse defenses against bacteria.

The results can explain why we itch first.

“It is very evolving of evolution”, which means that it is a behavior widely seen on the tree of life and displayed on many species, says Liwen DengImmunologist at the Harvard Medical School which was not involved in the new research. “But it seems so damaging, and we really had no understanding of what might be useful. This is really one of the first studies to show that this could be a beneficial answer,” said Deng à Live Science .

Despite these potential advantages, scratches still have negative effects, in particular with regard to chronic conditions. Gutting too much can worsen inflammation to the point that it slows down the healing process. In short, you can have too much good.

“The observation that scratch improves defense against Staphylococcus aureus suggests that this could be beneficial in certain contexts, “said Kaplan.” But the damage that scratching makes in the skin probably prevails over this advantage when itching is chronic. “”