Constantly scratching and picking at an itchy rash really does make it worse. But why? According to newly released research using mice as a model, scratching aggravates inflammation and swelling in a type of eczema called allergic contact dermatitis. The findings are detailed in a study published January 30 in the journal Science.Allergic contact dermatitisalso called contact dermatitisis an allergic reaction to various skin irritants and allergens. Common triggers include poison ivy, metals commonly found in jewelry like nickel, and ingredients in cosmetics or laundry detergent.While the signs and symptoms vary based on the allergen and individual, common symptoms of contact dermatitis include an itchy rash, leathery patches on the skin that are darker than usual, dry or cracked skin, bumps or blisters, swelling, burning, and tenderness. Get the Popular Science newsletter Breakthroughs, discoveries, and DIY tips sent every weekday. By signing up you agree to our Terms of Service and Privacy Policy.The urge to scratch these rashes can sometimes be irresistible, despite the fact that it can worsen symptoms and slow down healing time.Scratching is often pleasurable, which suggests that, in order to have evolved, this behavior must provide some kind of benefit, Dr. Daniel Kapan, a study co-author and dermatologist at the University of Pittsburgh said in a statement. Our study helps resolve this paradox by providing evidence that scratching also provides defense against bacterial skin infections.Mice in a cone of shameTo see what is driving this vicious cycle, the authors used itch-inducing allergens to induce eczema-like symptoms on the ears of two groups of mice. They used standard mice and a group that do not have the sensation to itch because they lack an itch-sensing neuron.When the normal mice were allowed to scratch, the team saw that their ears became swollen. Their ears also filled with inflammatory immune cells called neutrophils. By comparison, the inflammation and swelling appeared to be milder in the mice that lacked the itch-sensing neuron and normal mice that physically couldnt scratch because they were wearing tiny collars. These Elizabethan collars are similar to the cone that a dog might wear after a veterinary procedure. According to the authors, this experiment confirms that scratching further aggravates the skin.The team also found that scratching causes pain-sensing neurons to release a compound called substance P. The compound then activates mast cells. These are the key coordinators of inflammation that drive itchiness and inflammation in the skin.In contact dermatitis, mast cells are directly activated by allergens, which drives minor inflammation and itchiness, said Kaplan. 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 activated through two pathways.[ Related: These are the most common allergies (and the deadliest). ]Why mast cells are importantWhile mast cells are involved in a wide range of inflammatory skin conditions and allergic reactions, theyre important for protecting the skin against bacteria and other pathogens. The team were curious if scratching-induced activation of mast cells possibly affects the skin microbiome. A second experiment revealed that scratching reduced the amount of Staphylococcus aureus on the skin. This is the most common bacteria involved in skin infections.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.The team is now investigating new therapies for dermatitis that suppress inflammation by targeting receptors on mast cells. These treatments could potentially be used in several inflammatory skin conditions including rosacea and urticaria.