It's like a compulsion, just as irrepressible. Any skin itch triggers an urgent need to scratch, which makes the itch worse. It's a vicious circle, especially for those suffering from skin allergies such as contact dermatitis.
A team from the University of Pittsburgh (Pennsylvania, US) has uncovered the mechanics of this process in mice. The researchers used itch-inducing allergens to induce eczema-like symptoms in the ears of normal mice but also of mice that didn't scratch – either due to genetic modification removing their itch-transmitting neurons or because they wore a collar that prevented them from doing so.
In the skin, allergen molecules are captured by antibodies on the surface of mast cells, the key cells in allergic reactions.
When activated, mast cells release “pruritogenic” substances that stimulate certain neurons, triggering an irrepressible urge to scratch.
Scratching stimulates pain-sensitive neurons
These neurons release a small neuropeptide, “substance P,” which binds to another receptor on mast cells.
Mast cells, doubly activated by allergens and substance P, release massive quantities of tumor necrosis factor (TNF), which attracts other inflammatory cells (neutrophils).
At the same time, S. aureus antigens can also be captured by antibodies on the surface of mast cells. Scratching then helps reinforce the host's defenses against this bacterium.
Sources: Andrew W. Liu, Science, January 31, 2025
Le Monde infographics: Victoria Denys, Florence Rosier
The study shows that scratching aggravates inflammation by triggering a dual activation of mast cells, the immune cells that coordinate inflammatory processes. This process also notably recruits neutrophils, the soldier cells.
Mast cells are first activated by allergens such as nickel, copper, industrial glues and hair dyes, "which causes minor inflammation and itching," explained Daniel Kaplan, a professor of dermatology and immunology at the University of Pittsburgh, who coordinated this work. Then, in response to scratching, pain-sensitive neurons release substance P, a neuropeptide that in turn activates mast cells through a second pathway.
"The reason scratching triggers more inflammation in the skin is that mast cells are synergistically activated by these two pathways," said Kaplan. Eventually, they release massive quantities of TNF, a powerful messenger that coordinates inflammatory processes.
An entirely vicious circle? Not quite. The study also reveals an unexpected benefit of scratching: it boosts immune defenses against a bacterium, staphylococcus aureus (S. aureus), often present in large quantities on the skin of eczema patients – the bacterium most frequently implicated in skin infections. Scratching, the study shows, reduces the amount of S. aureus on the skin of mice.
"Scratching is often pleasurable, which suggests that, to have evolved, this behavior must provide some sort of benefit," explained Kaplan. "Our study helps to resolve this paradox by providing evidence that scratching also constitutes a defense against bacterial skin infections."
Translation of an original article published in French on lemonde.fr; the publisher may only be liable for the French version.