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Abstract: Delayed type hypersensitivity (DTH) reactions represent cell-mediated immune responses that exert important immunoprotective (resistance to viruses, bacteria, and fungi) or immunopathologic (allergic or autoimmune hypersensitivity) effects. We have used the skin DTH response as an in vivo model to study neuro-endocrine-immune interactions. We hypothesized that just as an acute stress response prepares the cardiovascular and musculoskeletal systems for fight or flight, it may also prepare the immune system for challenges (e.g., wounding) that may be imposed by a stressor (e.g., an aggressor). Studies showed that acute (2 hours) stress experienced before primary or secondary cutaneous antigen exposure induces significantly enhanced skin DTH. This enhancement involves innate as well as adaptive immune mechanisms. Adrenalectomy eliminates the stress-induced enhancement of DTH. Acute administration of physiological concentrations of corticosterone and/or epinephrine to adrenalectomized animals enhances skin DTH. Compared with those in controls, DTH sites from acutely stressed or hormone-injected animals show significantly greater erythema and induration, numbers of infiltrating leukocytes, and levels of cytokine gene expression. In contrast to acute stress, chronic stress is immunosuppressive. Chronic exposure to corticosterone or acute exposure to dexamethasone significantly suppresses skin DTH. These results suggest that during acute stress, endogenous stress hormones enhance skin immunity by increasing leukocyte trafficking and cytokine gene expression at the site of antigen entry. Elucidation of mechanisms mediating a stress-induced enhancement of skin immune function is important because such immunoenhancement can have protective (wound healing, resistance to infection) or pathological (allergic or autoimmune hypersensitivity) consequences.