Scratching a bug bite or rash offers only fleeting relief before trapping you in a cycle of prolonged itch and inflammation, according to researchers at the University of Pittsburgh.
Led by Daniel H. Kaplan, a professor of dermatology and immunology, scientists tracked the response in mice with allergic-contact dermatitis. They found that scratching activates mast cells, the body's alarm system. These cells release histamine, causing itchiness, and were triggered by a signaling molecule called substance P.
Mice wearing an Elizabethan-style collar, and therefore prevented from scratching, experienced significantly less inflammation.
Scientists then investigated why this counterproductive behavior felt good enough to survive evolution. A second experiment showed that scratching reduced levels of Staphylococcus aureus, a key bacteria linked to skin infections, on the skin's microbiome.
Kaplan noted this resolves a paradox: while damaging, the act offers an ancient bacterial defense. However, the researchers caution that in cases of chronic eczema, the tissue damage far outweighs the antimicrobial benefit. For temporary relief, dermatologists suggest cold compresses and hydrocortisone cream.