Everyone has experienced stress at one time or another, and it’s not hard to imagine that too much stress can make you more susceptible to sickness. But why? According to a recent study in the journal Biological Psychiatry, researchers believe they’ve found a mechanism that alters the body’s immune response when under stress. The researchers examined two groups — one composed of caregivers of family members with cancer and the other composed of individuals without that type of stress. They found that the gene expression of the caregivers’ monocytes — a type of white blood cell — had been altered so they were less responsive to cortisol — an anti-inflammatory hormone secreted by the adrenal glands to fight stress. “In other words, something goes awry in caregivers’ white blood cells so they are not able to ‘receive’ the signal from cortisol that tells them to shut down inflammation,” Gregory E. Miller, one of the study’s authors, wrote. This can increase the risk of illnesses such as depression, which is often associated with severe sleep disturbance. According to the researchers, many questions remain unanswered, like how stress produces the altered gene expressions, but they concluded the study is a vital step in understanding how stress affects the immune system.