Although the word stress generally has negative connotations, stress is a familiar aspect of life, being a stimulant for some but a burden for others. Numerous definitions have been proposed for the word stress. Each definition focuses on aspects of an internal or external challenge, disturbance, or stimulus; on perception of a stimulus by an organism; or on a physiological response of the organism to the stimulus (Goldstein and McEwen, 2002; McEwen, 2002; Sapolsky, 2004). Physical stressors have been defined as external challenges to homeostasis and psychological stressors as the "anticipation justified or not, that a challenge to homeostasis looms" (Sapolsky, 2005).An integrated definition states that stress is a constellation of events, consisting of a stimulus (stressor), which precipitates a reaction in the brain (stress perception), which activates physiologic fight-or-flight systems in the body (stress response) (Dhabhar and McEwen, 1997).The physiologic stress response results in the release of neurotransmitters and hormones that serve as the brain's alarm signals to the rest of the body. It is often overlooked that a stress response has salubrious adaptive effects in the short run (Dhabhar et al., 1995a; Dhabhar and McEwen, 1996, 2001) although stress can be harmful when it is longlasting (Irwin et al., 1990; McEwen, 1998; Sapolsky, 2004). An important distinguishing characteristic of stress is its duration and intensity. Thus, acute stress has been defined as stress that lasts for a period of a few minutes to a few hours and chronic stress as stress that persists for several hours per day for weeks or months (Dhabhar and McEwen, 1997). The intensity of stress may be gauged by the peak levels of stress hormones, neurotransmitters, and other physiological changes such as increases in heart rate and blood pressure, and by the amount of time for which these changes persist during and after the cessation of stress.

It is important to bear in mind that there exist significant individual differences in the manner and extent to which stress is perceived, processed, and coped with. These differences become particularly relevant in case of human subjects because stress perception, processing, and coping mechanisms can have significant effects on the kinetics and peak levels of circulating stress hormones and on the duration for which these hormone levels are elevated. The magnitude and duration of catecholamine and glucocor-ticoid hormone exposure in turn can have significant effects on leukocyte distribution and function (Dhabhar and McEwen, 2001; Pruett, 2001; Schwab et al., 2005).

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