FEB 07, 2025

Why Scratching an Itchy Rash Can Make it Worse

WRITTEN BY: Carmen Leitch

Scientists have now shown why it may not be a good idea to scratch an itchy rash, even though it feels good. This research used a mouse model of a kind of eczema known as allergic contact dermatitis, which is when irritants or allergens cause a swollen, itchy rash. When scratched, the swelling and inflammation gets worse, lengthening recovery times. The work has been reported in Science.

In this work, the mouse eczema model was used alongside mice that lacked the neurons that sense itchiness. Eczema was then induced on their ears. When the mice were allowed to scratch the itch, inflammatory cells known as neutrophils filled the area, leading to swelling and inflammation. If mice were prevented from scratching their itches with a physical collar, the levels of swelling and inflammation were lower compared to the scratching mice.

Additional work indicated that the act of scratching causes neurons that sense pain to secrete a molecule called substance P, which activates immune cells called mast cells. Mast cells can recruit neutrophils, and boost itchiness and inflammation.

"In contact dermatitis, mast cells are directly activated by allergens, which drives minor inflammation and itchiness," said Daniel Kaplan, MD, PhD, a professor of dermatology and immunology at the University of Pittsburgh.

"In response to scratching, the release of substance P activates mast cells through a second pathway, so the reason that scratching triggers more inflammation in the skin is because mast cells have been synergistically activated through two pathways," explained Kaplan.

While mast cells can cause problems like allergic reactions, they also help shield us from pathogens.

The work also showed that the microbiome of the skin can be affected by itching as well. Scratching was shown to reduce the levels of the most common skin bacterium -  Staphylococcus aureus.

This seems to suggest that scratching also has a benefit. By lowering bacteria levels, we may be protected from skin infections when we scratch. The researchers suggested that this may explain why scratching can feel so satisfying - it can be helpful.

"The finding that scratching improves defense against Staphylococcus aureus suggests that it could be beneficial in some contexts," said Kaplan. "But the damage that scratching does to the skin probably outweighs this benefit when itching is chronic."

Sources: University of Pittsburgh, Science