Vagal immune-to-brain communication: a visceral chemosensory pathway

The immune system operates as a diffuse sensory system, detecting the prese nce of specific chemical constituents associated with dangerous micro-organ isms, and then signalling the brain. In this way, immunosensation constitut es a chemosensory system. Several submodalities of this sensory system func tion as pathways conveying immune-related information, and can be classifie d as either primarily brain barrier associated or neural. The vagus nerve p rovides the major neural pathway identified to date. The initial chemosenso ry transduction events occur in immune cells, which respond to specific che mical components expressed by dangerous micro-organisms. These immune chemo sensory cells release mediators, such as cytokines, to activate neural elem ents, including primary afferent neurons of the vagal sensory ganglia. Prim ary afferent activation initiates local reflexes (e.g. cardiovascular and g astrointestinal) that support host defense. In addition, at least three par allel pathways of ascending immune-related information activate specific co mponents of the illness response. In this way, immunosensory systems repres ent highly organized and coherent pathways for activating host defense agai nst infection. (C) 2000 Elsevier Science B.V. All rights reserved.