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The classic biomedical view is that stress hormone effects on the immune system are largely pathological, especially if the stress is chronic. However, more recent interpretations have focused on the potential adaptive function of these effects. This paper examines stress response-immune system interactions from a physiological network perspective, using insects because of their simpler physiology. For example, stress hormones can reduce disease resistance, yet activating an immune response results in the release of stress hormones in both vertebrates and invertebrates. From a network perspective, this phenomenon is consistent with the 'sharing' of the energy-releasing ability of stress hormones by both the stress response and the immune system. Stress-induced immunosuppression is consistent with the stress response 'borrowing' molecular components from the immune system to increase the capacity of stress-relevant physiological processes (i.e. a trade off). The insect stress hormones octopamine and adipokinetic hormone can also 'reconfigure' the immune system to help compensate for the loss of some of the immune system's molecular resources (e.g. apolipophorin III). This view helps explain seemingly maladaptive interactions between the stress response and immune system. The adaptiveness of stress hormone effects on individual immune components may be apparent only from the perspective of the whole organism. These broad principles will apply to both vertebrates and invertebrates., (Copyright © 2016 Elsevier Inc. All rights reserved.)

A whole organism, network approach can help explain the adaptive purpose of stress-induced changes in immune function. In insects, mediators of the stress response (e.g. stress hormones) divert molecular resources away from immune function and towards tissues necessary for fight-or-flight behaviours. For example, molecules such as lipid transport proteins are involved in both the stress and immune responses, leading to a reduction in disease resistance when these proteins are shifted towards being part of the stress response system. Stress responses also alter immune system strategies (i.e. reconfiguration) to compensate for resource losses that occur during fight-or flight events. In addition, stress responses optimize immune function for different physiological conditions. In insects, the stress response induces a pro-inflammatory state that probably enhances early immune responses., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Stress-induced changes in immune function occur in animals across phyla, and these effects are usually immunosuppressive. The function of this immunomodulation remains elusive; however, the existence of specialized receptors on immune cells suggests that it is adaptive. A comparative approach may provide a useful perspective. Although invertebrates have simpler endocrine/neuroendocrine systems and immune systems than vertebrates, they have robust stress responses that include the release of stress hormones/neurohormones. Stress hormones modify immune function in mollusks, insects, and crustaceans. As in vertebrates, the effects of stress hormones/neurohormones on invertebrate immune function are complex, and are not always immunosuppressive. They are context-, stressor-, time- and concentration-dependent. Stress hormone effects on invertebrate immune function may help to re-align resources during fight-or-flight behavior. The data are consistent with the hypothesis that stress hormones induce a reconfiguration of networks at molecular, cellular and physiological levels that allow the animal to maintain optimal immunity as the internal environment changes. This reconfiguration enhances some immune functions while suppressing others. Knowing the molecular details of these shifts will be critical for understanding the adaptive function of stress hormones on immune function., (Copyright © 2012 Elsevier Inc. All rights reserved.)