ify the physical mechanisms that could account for immune suppression when an animal is exposed to uncontrollable shock. Although it had long been known that stress elevates levels of the adrenal gland hormone cortisol, which is itself immunosuppressive, early studies ruled out cortisol as the only major player in uncontrollable stress-induced immunosuppression (Keller, Weiss, Schleifer, Miller, & Stein, 1983). Later studies succeeded in locating brain regions involved in the uncontrollable stress-induced immune suppression. Subjecting an animal to uncontrollable, unpredictable shock activates an area of the brain called the periaqueductal gray area, which mediates immune suppression (Demetrikopoulos, Siegel, Schleifer, Obedi, & Keller, 1994; Weber & Pert, 1989). Activation of the periaqueductal gray is associated with behavioral manifestations, namely, the behavioral manifestations of learned helplessness (Mater et al., 1993). Squealing, defensive aggression, and defensive freezing are further observed when the periaqueductal gray of a rodent is activated (Mater et al., 1993). An anthropomorphic interpretation of the rodent's experience is that the rodent feels vulnerable. Many types of distressing events that human beings encounter appear to share essential features with the laboratory procedure of uncontrollable shock. A wide variety of stressful events are known to induce immune suppression in people. Medical students undergoing examinations exhibit immune suppression (Kiecolt-Glaser et al., 1984; Kiecolt-Glaser et al., 1986), as do caregivers of Alzheimer's patients (Kiecolt-Glaser et al., 1987), people who have lost a loved one (Bartrop, Luckhurst, Lazarus, Kiloh, & Penny, 1977), and women who are unemployed (Arnetz et al., 1987). People who experience greater daily stress show immune system depression (Stone et al., 1994). Depressed individuals, particularly those who are older or more severely depressed, are likely to be immunos...
