The urge to scratch an itch may have evolved for a reason, but we have struggled to pinpoint whyCavan Images/AlamyForget what your parents told you, scratching an itch may be good for you, by boosting your bodys defence against bacteria.Scratching is a paradoxical phenomenon. It can feel nice to run your nails over an itchy bit of skin, which hints that there is an evolutionary reason we get pleasure from doing it, and yet it can also aggravate eczema and rashes or reopen a wound. AdvertisementTo dig into this dichotomy, Dan Kaplan at the University of Pittsburgh, Pennsylvania, and his colleagues used allergens called haptens to induce eczema-like symptoms on mices ears.The ears of mice that could scratch at will swelled up more and contained more inflammatory cells called neutrophils which help our body fight infection than those wearing a collar that stopped them reaching their ears.To see how this scratching was affecting the skin, the researchers looked into what was going on inside the cells. They found that the scratching made pain-detecting neurons release a chemical known as substance P, which then activated white blood cells called mast cells, encouraging the body to produce more neutrophils. Get the most essential health and fitness news in your inbox every Saturday.Sign up to newsletterIt shows that the act of scratching is really important in augmenting mast cell behaviour, says Holly Wilkinson at the University of Hull, UK.The team also found that scratching reduced the amount of Staphylococcus aureus, a bacterium commonly involved in skin infections, on the mices skin. This microbe can trigger a lot of skin inflammation, which manifests as dermatitis, says Kaplan. About 24 hours of scratching is enough to change the microbiome in measurable ways, he says.However, by activating the mast cells, the allergens also triggered a pathway that incited inflammation, showing that in conditions like eczema, a type of dermatitis, the body is effectively getting a double dose of skin inflammation, which is when scratching will probably lead to increased damage.If youre basically healthy and scratching, it is a good thing, says Wilkinson. But there is that tipping point and if you are scratching too much then it could become a negative thing.Kaplan says his team has done preliminary work showing similar benefits from scratching in people.Journal reference:Science DOI: 10.1126/science.adn9390Topics:immune system