The neuroendocrine-immune axis.

The three different systems that contribute to development and maintenance of homeostasis, the nervous, endocrine and immune systems are closely interrelated. First, it was established that the neuroendocrine system was able to modulate the immune response. More recently, it has been shown that the immune system is able to function as a neuroendocrine organ, since it can synthesize not only hormones and neuropeptides, but also cytokines that have an impact on the neuroendocrine system. This has led to the demonstration of the existence of cytokine receptors within the nervous system. At the same time, various cytokine effects on the nervous system have been described including growth-promoting activity, neuromodulatory action, fever induction, sleep and decreased food intake. The latest element in this network of intricate interrelationships has been the demonstration of cytokine production by cells of the nervous system. Moreover, it appears that the brain is no longer the immunologically deprived site it was once thought to be, and that all phases of an immune response can potentially occur within the confines of the BBB.