Because there are individual differences in what is considered stressful, psychologists distinguish between stressors (the object, situation or person that causes a state of stress) and the stress-response (the physiological, emotional and behavioural effects).

stress has been shown to have a negative effect on the immune system in many studies; for example, Kiecolt-Glaser et al (1984) found that exam stress reduced the function of important immune cells; other studies have shown that wounds are slower to heal when people are suffering from stress. Now new research by Rita Effros, professor of pathology and laboratory medicine at the David Geffen School of Medicine at UCLA, and a member of the Jonsson Cancer Center, Molecular Biology Institute and UCLA AIDS Institute may explain how stress ages the immune system.