There is a direct link between prolonged stress and a weaker
immune system, resulting in higher susceptibility to illness and disease, according to a study conducted by researchers at the David Geffen School of Medicine at UCLA. The study showed that cortisol, released when the “fight or flight” response is stimulated, wears down the immune system if it remains in the bloodstream for long periods of time.

Cortisol weakens the immune system through its impact
on telomerase, an enzyme within immune cells that keeps them young. When telomerase is not produced, the lifespan of individual immune cells is shortened and the number of times they can reproduce declines.The study was published in the May
issue of Brain, Behavior and Immunity (2008, 22; 600–605).