Peptides as regulators of the immune system: emphasis on somatostatin.

Study of the communication between nervous and immune systems culminated in the understanding that cytokines, formerly considered exclusively as immune system-derived peptides, are endogenous to the brain and display central actions. More recently, immune cells have been recognized as a peripheral source of "brain-specific" peptides with immunomodulatory actions. This article reviews studies concerning reciprocal effects of selected cytokines and neuropeptides in the nervous and immune systems, respectively. The functional equivalence of these two categories of communicators is discussed with reference to the example of the actions of neuropeptide somatostatin in the immune system.