Neuroendocrine factors alter host defense by modulating immune function.

An increasing body of evidence demonstrates that there is bidirectional communication between the neuroendocrine and immune systems. Interaction between these systems results in a variety of outcomes, including the well documented "sickness behavior" elicited by cytokines of the immune system that can enter the brain and activate second messengers that modify neuronal activity. Crosstalk between the neuroendocrine and immune systems can also result in production of factors by the nervous and endocrine systems that alter immune cell function and subsequent modulation of immune responses against infectious agents and other pathogens. Continued exposure to molecules produced by the neuroendocrine system has also been known to increase susceptibility and/or severity of disease. Furthermore, neuroendocrine factors are thought to play a major role in gender-specific differences in development of certain disorders, including autoimmune/inflammatory diseases that have a two to tenfold higher incidence in females compared to males. Neuroendocrine factors can affect immune cells at the level of gene transcription but have also been shown to modify immune cell activity by interacting with intracellular molecules, resulting in modified ability of these cells to mount a potent immune response. In this review, we will consider various effects of the neuroendocrine system and its proteins on specific populations of immune cells and associated responses in host immunity against pathogens. We will further discuss how this modification of immune cell activity by the neuroendocrine system can contribute to susceptibility/severity of disease development.