PITTSBURGH — Research conducted by Dr. Daniel Kaplan's lab at the University of Pittsburgh indicates that scratching can intensify skin inflammation by activating mast cells through the chemical substance P. Normal mice with an irritant applied to their ears scratched the site, which led to inflammatory immune cells accumulating and increased swelling. Conversely, normal mice that were fitted with collars to prevent scratching showed reduced swelling and fewer inflammatory cells.

Dr. Kaplan's team reported findings that substance P, a chemical messenger released by pain-sensing nerve cells, can activate mast cells through a molecular pathway distinct from allergens. Mast cells, white blood cells in the immune system, release compounds, including histamine, that can contribute to allergic reactions and inflammation. Pain signals can also activate mast cells.

To investigate further cellular responses, the researchers used collars on mice to prevent scratching. Mice bred with defective itch-sensing nerve cells developed a milder rash after irritant exposure. Kaplan's team also infected mice with Staphylococcus aureus bacteria and repeated the experiment with the anti-scratching collars, finding that mice allowed to scratch had lower levels of the bacteria on their ears.

Dr. Kaplan, a dermatologist at the University of Pittsburgh, stated that ignoring a mosquito bite causes the itch to disappear in five or 10 minutes for most people. "But if you start scratching it, it's your friend for a week," Kaplan said, referring to the intensified itchiness and inflammation that follows scratching. He added, "Ultimately, scratching is deleterious."

Why It Matters

This research identifies a mechanism by which scratching can worsen skin conditions, linking mechanical irritation to a specific immune response involving substance P and mast cells. The findings suggest that interventions targeting this pathway or preventing scratching could help manage inflammatory skin conditions.