If scratching an itch is bad for us, why does it feel so good? Researchers at the University of Pittsburgh (USA) studied allergic contact dermatitis in mice to understand the consequences of scratching on the rash.Scratching is often a pleasurable experience, suggesting that, to have evolved, this behavior must provide some benefit, says Daniel Kaplan, lead author of the study. Our study helps resolve this paradox by providing evidence that scratching also provides a defense against bacterial skin infections."These results were published in the prestigious journal Science.
Scratching feels good, temporarily
First, where does the itching sensation come from? It is caused by nerve endings distributed on the skin: pruriceptors. The information passes through several neurons before reaching the brain and causing the urge to scratch. Now, let's imagine that we take action. This behavior relieves, at least temporarily, the itching. Another type of receptor is involved: nociceptors, specialized in pain.
This new information blocks the itch information at the spinal cord level, which causes the feeling of relief. But in return, the inflammation intensifies, the symptoms worsen and healing takes longer. To understand the drivers of this vicious circle, Daniel Kaplan's team used the mouse model.
An increase in inflammation
The researchers used allergens to trigger contact dermatitis, causing itchy ears in mice.Allergic contact dermatitis is a reaction to allergens or irritants, including poison ivy and certain metals such as nickel. It causes an itchy, swollen rash,” the study authors said in a press release. Some of the rodents studied were able to scratch themselves. Others wore cones, like those dogs wear after a visit to the vet.
The third group lacked pruriceptors. The result: In the first group of mice, allowed to scratch, their ears swelled and an influx of immune cells, called neutrophils, fueled the inflammation.In contrast, inflammation and swelling were much milder in mice that could not scratch or lacked pruriceptors," note the researchers.
In response to scratching, pain-sensitive neurons release a compound called substance P. This molecule then attaches to specific receptors and "activates" mast cells, immune cells. They cause itching and inflammation by recruiting neutrophils. Two pathways therefore stimulate the inflammatory reaction, which becomes more pronounced. Symptoms worsen and healing takes longer. However, scratching provides a feeling of pleasure, so researchers have tried to identify the benefit that could be gained from this practice.
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But better protection against bacteria
“Mast cells are responsible for many inflammatory skin diseases, but they also play an important role in protection against bacteria including," they explain. Could it be that scratching affects the skin microbiome? Their experiments show a lower quantity of Staphylococcus aureus, the most common bacteria involved in skin infections.These results show that scratching improves defense against the Staphylococcus aureus, which suggests that it might be beneficial in some contexts”, says Daniel Kaplan.But the damage to the skin from scratching surely outweighs this benefit when the itching is chronic.”
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Today, their research focuses on studying new therapies for dermatitis and other inflammatory skin diseases, such as rosacea and hives. The goal: to reduce inflammation by targeting mast cell receptors.