ABSTRACT
Although the immunosuppressive effect of chronic stress has been established, a stress response that downregulates the whole immune system does not make biological sense, especially if an animal has to endure difficult times in which there is also increased infection risk. At high animal densities, animals are faced simultaneously with food restriction (FR), social conflict (SC), and greater parasite-pathogen exposure. We hypothesized that the stress response to chronic stressors that covary with infection risk is not entirely immunosuppressive. Our prediction was that a chronically stressed animal would respond by enhancing innate defenses, while reducing investment in acquired immunity. In a laboratory setting, rats were exposed to prolonged FR and/or SC, and natural and specific antibody levels were repeatedly measured. Our prediction was fulfilled only partly, as FR and SC interacted to enhance natural antibodies, but rats exposed to either or both stressors also showed significantly higher levels of specific antibodies. These results suggest that, in the rat, chronic stress results in a prioritization of both innate and acquired humoral defenses, which makes biological sense provided the stressors examined usually signal an increased infection risk.