Extract: There is an extensive body of scientific literature related to the field of research called psychoneuroimmunology. Clinical studies, backed up by mechanism studies, have provided convincing evidence that the central nervous system (CNS) interacts with the endocrine and immune systems and that these interactions are bi-directional. Stress has been a focal point in this body of literature because it is known that stress can induce immune dysregulation across many aspects of the humoral and cellular immune responses. These studies have dated back to the 1960s and 1970s, and have included some very elegant studies involving animal models. The important outcome of this research is that stress-induced immune dysregulation can produce changes that are not only statistically significant but, most importantly, biologically significant in terms of health risk. It is now well established that there are very complex bi-directional interactions between the CNS and the immune system mediated by the endocrine system. Two important aspects of these interactions include the production of stress hormones by the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic-adrenal-medullary (SAM) axis. The interactions between immune cells also take place through the production of cytokines. Hormones can modulate immune function by binding to their receptors, which are expressed on virtually every type of immune cell. The modulation of cytokines has been shown to feedback to the brain, producing changes in the HPA axis, as well as inducing sickness behavior such as fever, loss of appetite, changes in sleep patterns and depression.