Few experiences are as unbearable as an itch that sets in. And conversely, few pleasures are as immediate as that of finally giving in to scratching. Despite its annoying appearance, itching plays a biological role: it serves as an alarm signal. When an irritating substance (a parasite, an allergen, or other) comes into contact with the skin, specific nerve receptors activate and send a signal to the brain. It is a protective mechanism, just like pain.
In recent years, scientists have focused more on the psychological dimension of these behaviors, reports the BBC. Because even without a real threat, it is possible to feel itchy. This phenomenon can even be “contagious”. In 2011, dermatologist Gil Yosipovitch demonstrated this by asking healthy volunteers and patients with atopic dermatitis (a chronic inflammatory skin disease caused by an abnormal immune response and a deficiency in the skin barrier) to watch videos of people scratching. Whether they received an injection of histamine (which causes itching) or a simple saline solution, all guinea pigs reported an increase in itching sensations.
A survival reflex
This phenomenon is not limited to humans. Two years after his experiment, Gil Yosipovitch demonstrated that macaques also started to scratch by watching a video of other monkeys subject to itching. There is also a study suggesting that we humans, as well as other mammals, are programmed to avoid people who scratch.
It was the clinician and neuroimmunologist Brian Kim, from the Icahn School of Medicine at Mount Sinai in New York (United States), who observed this striking contrast in the laboratory: when a mouse sees a conspecific scratching, it tends to move away from it. On the other hand, when faced with a mouse that is clearly in pain, it approaches and adopts caring behaviors, such as licking or grooming.
This reaction is explained by the protective role of scratching, which helps eliminate parasites and mites, thus reducing the risk of infections. Observations made in retirement homes, where immobility promotes scabies epidemics, confirm this role. Paradoxically, scratching can therefore also serve as an involuntary social signal. A person who scratches a lot can indicate to those around them that they are potentially carrying a contagious infection. “I think instinctively it’s a question of survival”concludes Brian Kim. For people suffering from chronic itching, this can cause significant psychological suffering. Many patients report feelings of shame and embarrassment that can lead to anxiety or depression.
