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A new study published in the journal Science has uncovered why scratching an itchy rash may be more harmful than previously thought. The research sheds light on the physiological processes triggered when scratching aggravates inflammation and worsens symptoms in a mouse model of allergic contact dermatitis (ACD), a type of eczema.
ACD is an allergic reaction to allergens or skin irritants—such as poison ivy or metals like nickel—that results in an itchy, inflamed rash. While scratching provides immediate relief, it often exacerbates the condition by increasing inflammation and slowing the healing process.
Senior author Dr. Daniel Kaplan, a professor of dermatology and immunology at the University of Pittsburgh, explained, “At first, these findings seemed to introduce a paradox: If scratching an itch is bad for us, why does it feel so good?” He added, “Scratching is often pleasurable, which suggests that, in order to have evolved, this behavior must provide some kind of benefit. Our study helps resolve this paradox by providing evidence that scratching also provides defense against bacterial skin infections.”
To investigate the effects of scratching, Kaplan and his team, including first author Andrew Liu, a student in the Medical Scientist Training Program at Pitt, conducted experiments on mice with eczema-like symptoms. They compared the behavior of normal mice with those that lacked the itch-sensing neuron, making them unable to scratch.
When normal mice were allowed to scratch, their ears became more swollen and filled with inflammatory immune cells called neutrophils. In contrast, inflammation was less severe in mice that could not scratch due to wearing Elizabethan collars or lacking the itch-sensing neuron. This experiment confirmed that scratching exacerbates the inflammatory response in the skin.
Further investigation revealed that scratching activates pain-sensing neurons to release a compound called substance P. This, in turn, activates mast cells, which are crucial in coordinating inflammation and driving both itchiness and swelling in response to allergens. Kaplan explains, “In contact dermatitis, mast cells are directly activated by allergens, which drives minor inflammation and itchiness. In response to scratching, the release of substance P activates mast cells through a second pathway.”
While mast cells contribute to inflammation, they also play an important role in defending against bacteria and pathogens. To explore this, the researchers analyzed the skin microbiome and found that scratching reduced the amount of Staphylococcus aureus, a bacteria often linked to skin infections. Kaplan noted, “The finding that scratching improves defense against Staphylococcus aureus suggests that it could be beneficial in some contexts. But the damage that scratching does to the skin probably outweighs this benefit when itching is chronic.”
The researchers are now looking into potential therapies for treating dermatitis and other inflammatory skin conditions, such as rosacea and urticaria, that aim to suppress inflammation by targeting mast cell receptors.
Disclaimer: The information in this article is based on a study published in the journal Science. While the findings provide valuable insights, further research is needed to confirm these results in human models and to assess the clinical implications for treating inflammatory skin conditions. Always consult with a healthcare professional for advice tailored to your specific condition.
