Stress and immunity in wild vertebrates: timing is everything.

Stress has profound effects on vertebrate immunity, but most studies have considered stress-immune interactions in terms of wild animals enduring demanding, but predictable activities (e.g., immune alterations during breeding). A growing biomedical literature, however, indicates that stress may not be obligatorily immunosuppressive; in response to transient, unpredictable stressors, immune activity can be enhanced, especially in body areas requiring immune protection. Also, immune sensitivity to stressors is not fixed throughout life; oftentimes, glucocorticoid (GC) insensitivity can be induced. Further GC sensitivity can be programmed early in life; greater exposure to stressors prior to maturity heightens GC effects on immunity in adulthood. In the present paper, I review the cellular and molecular mechanisms that link stress responses to immune adjustments over short time scales in domesticated species then I attempt to place stress-immune interactions in a naturalistic, organismal context. When, how and why stressors affect immunity in wild animals remains practically unstudied.