Imagine you have a motion detector in your backyard to alert you to intruders. But the sensitivity on the sensor is too high, so even harmless things (a squirrel, a fly) set it off. When you have rosacea, your immune system is like that alarm, says Richard Gallo, M.D., Ph.D., chair of dermatology at UC San Diego School of Medicine. A properly functioning immune system fights harmful substances. But in people with rosacea, it overreacts to certain stimuli, causing your face to develop redness or darkening, bumps, and/or visible blood vessels. But why does this happen in the first place? We’ve got answers.