Conditioning Immune and Endocrine Parameters in Humans: A Systematic Review

BACKGROUND: Conditioned pharmacological effects may provide relevant clinical opportunities to improve treatment for patients with a variety of conditions. The aim of this systematic review was to create an overview of studies in this field of research and to investigate whether specific characteristics of the study design make for successful conditioning.

METHODS: The protocol of this review was registered in Prospero (PROSPERO 2015: CRD42015024148). A systematic literature search was conducted in the databases PubMed, Embase, and PsychInfo. Studies were included if they were placebo-controlled trials in humans in which the effects of a pharmacological agent on immune or endocrine outcomes (e.g., interleukin-2 and cortisol) were conditioned, using a specific conditioned stimulus. The risk of bias of each study was assessed using the Cochrane risk-of-bias tool.

RESULTS: The final selection included 16 studies. Overall, those studies indicate that conditioning of immunosuppression, conditioning of allergic responses, and conditioning of insulin and glycemic responses is possible. Regarding immunostimulants, antiallergic effects, and cortisol conditioning, the preliminary results are promising, but additional studies are needed.

CONCLUSIONS: This systematic review shows classical conditioning of immune and endocrine responses for various pharmaceutical substances. The studies reviewed here indicate that the number of acquisition and evocation sessions, and characteristics of the unconditioned and conditioned stimuli, are important determinants of the effectiveness of pharmacological conditioning on immune and endocrine parameters. In the future, conditioned pharmacological effects may be used clinically as adjunct therapy in various patient populations.