Constantly scratching and picking at an itchy rash really does make it worse. But why? According to newly released research using mice as a model, scratching aggravates inflammation and swelling in a type of eczema called allergic contact dermatitis.

Despite the saying “happiness is having a scratch for every itch,” itching often makes the itchy skin condition worse. So why is the instinct so strong? In a study with mice, researchers from the University of Pittsburgh uncovered why scratching worsens inflammation and swelling—and why humans and other animals may have,

Mice that were free to scratch their itchy ears saw the number of immune cells go up and levels of infection-causing bacteria decline

Scratching isn’t just a bad habit — it’s an immune defense mechanism. While it fuels inflammation, it also helps fight infections by drawing immune cells to the skin. Scientists have uncovered how this paradoxical process works,