Grandmothers and mothers are partly right every time they yell at us: "Don't scratch, it's worse!". They do not possess the scientific knowledge behind the prohibition of that gesture, but they know it is harmful. No one listens. That itch that only calms down if one scratches well. And the more the gesture is repeated, time and time again, each time more insistently, the more pleasure is obtained, despite being accompanied by the unease of the sensation and the damage we do to the skin.
A new study published in the latest issue of the journal Science reveals the dual nature of the vicious circle: although it can worsen dermatological inflammation, it also plays a role in stimulating immune defenses against bacterial infections at the site of the injury. In other words, we now have a reason to counter our grandmothers' "don't scratch" advice: "I do it to improve the defenses of my skin against external aggressors."
Daniel Kaplan, lead author and professor of Dermatology and Immunology at the University of Pittsburgh, states that they seek to resolve the paradox: "If scratching is bad, why does it feel so good?".
Kaplan explains that if friction is usually pleasurable, "it is suggested that there is an evolutionary reason behind this behavior by providing some kind of benefit." The Pittsburgh researcher states that their study helps resolve the paradox "by providing evidence that rubbing also provides defenses against bacterial dermatological infections."
Scratching is a natural and instinctive response to the sensation of itching. It plays a key role in many skin conditions and injuries, such as dermatitis and insect bites, where chronic itching can be a significant source of discomfort. Repetitive friction can worsen the damage by promoting inflammation in a "cycle of itching-scratching" that perpetuates itself, where scratching intensifies the itching and can worsen the injury.
An example is found in allergic contact dermatitis, which is a reaction to skin allergens or irritants - including poison ivy and certain metals like nickel - that causes an itchy and inflamed rash. Succumbing to the often irresistible urge to scratch triggers increased inflammation that worsens symptoms and slows down the healing process for up to 6% of the population affected.
However, unlike pain, which is accompanied by an aversion response, scratching can be pleasurable, leading to that adaptive benefit. To find out what drives this vicious circle, Kaplan along with Andrew Liu, a student in the Pitt Medical Scientist Training Program, and their team used itch-inducing allergens to induce symptoms similar to eczema in the ears of normal mice and those that do not itch because they lack the itch-sensing neuron.
Using a novel genetically modified mouse model, they explored how eliminating the function of non-peptidergic itch-sensing neurons 2 affects the connection between itching, scratching, and inflammation. Liu's team discovered that scratching activates pain-sensitive neurons that release substance P, which stimulates mast cells to increase inflammation, mainly by attracting neutrophils. However, although friction would exacerbate issues like dermatitis, it would also aid the individual's immune defense by reducing bacteria, such as Staphylococcus aureus, during infections.
Furthermore, that scratch can influence the skin microbiome at the site of the injury, potentially preventing microbiota imbalances, although chronic conditions like atopic dermatitis complicate this. According to the authors, the findings suggest that scratching serves as both a pathological driver of inflammation and an evolutionary mechanism to reinforce protection against infections.
"Beyond defining an unidentified neuroimmune itch-scratching circuit, Liu's findings and his colleagues lay the groundwork for knowledge to help people suffering from chronic itching," writes Aaron Ver Heul from the University of Washington in an article accompanying the study's paper in Science.
These findings, as Ver Heul concludes, shed light on a pharmacologically targeted pathway that explains how scratching triggers inflammation, resolving the paradox that rubbing is both a harmful pathological process and a beneficial evolutionary adaptation.