The efferent pathway hypothesis-A mini-review on conditioned immune enhancement

Conditioned immune responses demonstrate that learned sensory cues can modulate peripheral immunity without re-exposure to the original immunological trigger. In 2026, this research area reaches the centennial of early Pavlovian immune-reflex experiments that already reported conditioned leukocyte shifts and enhancement-like resistance to infection. Although modern psychoneuroimmunology has been shaped largely by conditioned immunosuppression, conditioned immune enhancement remains comparatively less explored despite its relevance to anticipatory host defense, immune surveillance, tumor biology, and neuroimmune regulation. This mini review focuses on the efferent, or recall, pathways of conditioned immune enhancement and separates them from afferent acquisition signals and central cue-immune-state representations. Classical odor-conditioning paradigms using camphor and the viral mimic polyinosinic:polycytidylic acid identified interferon-β as an acquisition-related signal, whereas recall studies implicated β-endorphin, μ-opioid receptor, glutamatergic/NMDA, monoaminergic, catecholaminergic, cholinergic, serotonergic, ACTH-related, interferon-α-related, and endocrine mechanisms. Conditioned enhancement has been shown for natural killer cell activity, cytotoxic T-lymphocyte responses, neutrophil activity, antibody responses, and tumor-model readouts. We further integrate recent circuit-level studies of insular immune-state retrieval, brain-to-spleen humoral control, vagal cytokine coding, and descending sympathetic inflammatory pathways, together with transcriptomic data from a gene-agnostic multi-tissue pilot study assessing post-recall gene-expression dynamics. Together, these findings argue against a simple hypothalamic-pituitary-adrenal axis model and instead support a temporally organized, multi-channel efferent architecture. Dissecting and understanding this emerging architecture may provide the mechanistic basis for translating the efferent arm of conditioned immune enhancement into future therapeutic concepts with clinical impact.