Itching and inflammation: why it's best not to scratch a rash
A recent study by scientists at the University of Pittsburgh, published in the journal Science, helps us understand why scratching an itchy rash aggravates eczema and slows healing, even though it itself provides temporary relief.
The experiment was conducted on a mouse model of allergic contact dermatitis, a type of eczema caused by a reaction to irritants (such as poison ivy or metals like nickel).
Source: A.W. Liu et al. "Scratching promotes allergic inflammation and host defence via neurogenic mast cell activation," Science (2025). DOI: 10.1126/science.adn9390
The authors of the paper, led by Daniel Kaplan, professor of dermatology and immunology, faced a seeming contradiction: if "scratching" is harmful, why does this process often seem so pleasurable and even evolutionarily fixed? The researchers found the answer in the dual role of inflammation:
Eczema exacerbation.
In the experiment, they induced symptoms similar to human contact dermatitis in mice and then allowed the animals to scratch or, conversely, prevented them from doing so. It turned out that in the case of uncontrolled scratching the subjects had more severe swelling and a significant accumulation of neutrophils (immune cells). The mice without the ability to scratch (thanks to the special collar) had much less inflammation.Additional protection against bacteria.
Paradoxically, scratching can have a positive effect on the skin's defence against infection. Scientists have found that pain neurons release substance P (substance P), which activates mast cells - important elements of the immune system that can deal with harmful bacteria. In particular, brushing reduced the number of Staphylococcus aureus, one of the most common causative agents of skin infections. This may explain why we experience mechanical stimulation of the skin as "beneficial" or pleasurable.
As Kaplan explains, if it is a short-term effect, minor skin damage may be justified for the sake of pathogen control. However, for chronic itching or eczema, scratching leads to excessive inflammation, exacerbates symptoms and slows down the healing process.
The research team now plans to use the findings to develop new therapies for dermatological diseases - such as rosacea and urticaria. The aim is to "switch off" the excessive inflammatory response without depriving the body of its natural defences.
Dr Kaplan emphasises that despite the apparent benefits, brushing for prolonged itching harms the skin more than it helps. For people with eczema or other forms of dermatitis, it is important to look for alternative ways to reduce irritation and consult with specialists about anti-inflammatories.