["Sickness behavior"--mechanisms of origin and significance]

Psychoneuroimmunology, a combination of immunology and neurobiology, is a new field that has emerged over the past 20 years. There is now overwhelming evidence suggesting that the central nervous system is capable of regulating the immune response via two pathways--the autonomic and the neuroendocrine. It is also well documented that the immune system can influence the central nervous system by regulatory molecules or cytokines produced by activated immune cells. In this way, there is a bi-directional communication pathway between the two systems. Sickness behavior seems to present excellent evidence for the existence of immune-system-brain communication. Nonspecific symptoms of infection and inflammation include not only profound physiological but also behavioral changes. Behavioral changes can include anorexia, adipsia, increased sleepiness and depression in social, sexual exploration and general activity. These behavioral changes triggered by pro-inflammatory cytokines are thought to have evolved to conserve the energy necessary to fight infection. They may also minimize infection of conspecifics and reduce susceptibility to predation. Taken together, sickness behavior represents a behavioral reorganization highly motivated to promote survival and recovery. Knowing the mechanisms of neuroimmunomodulation allow us to better understand both the behavioral changes associated with immunotherapy and the changes in immune activation in major depression and other mental disorders.