The immune system and integrated homeostasis.

The immune system is usually viewed as an effector mechanism reacting to environmental antigenic challenge with defensive responses designed to eliminate 'foreign' substances and return to standby or surveillance mode. However, there is now substantial evidence to suggest that immunity is not effector-biased but is also a sensory organ and forms part of an integrated homeostatic network. Evidence that communication pathways exist between the immune and central nervous systems which support bi-directional information flow, and that the output of the immune system influences other physiological adjustments to environmental change, is discussed in this paper. The proposition that an endocrine-immune gradient determines the homeostatic response to combined microbial and other ambient stressors is established and the potential for management of this gradient is explored.