As we wrote in our book Demystifying Psychiatry, psychiatry is the “medical specialty that deals with disorders affecting the human mind and behavior.” Psychiatrists are interested in the mechanisms by which the brain generates normal and abnormal behaviors. Neuroscience-oriented psychiatrists often talk about neurotransmitters, neural systems, drugs that influence the brain, and the relationship between psychiatric disorders and brain function.

Immunologists specialize in the normal and abnormal function of the immune system. This system is designed to protect organisms, including humans, from disease. The human immune system involves a large variety of chemicals produced by specific immune system cells that combat bacteria, viruses, and other pathogens that can lead to dysregulation of various bodily functions. Sometimes, the immune system malfunctions, which can lead to a variety of autoimmune illnesses (meaning that the body’s immune system attacks itself). Rheumatoid arthritis and lupus are two examples of common autoimmune diseases.

Although immunology and psychiatry are very different fields, increasing evidence indicates that substances generated by immune system cells are actively involved in regulating brain function. The importance of this line of inquiry was highlighted recently by the July 2016 issue of the journal Biological Psychiatry, entitled “Inflammation and Immune Mechanisms in Neuropsychiatry.” In that issue, leaders in the field discussed the role of immune system dysfunction in a variety of neuropsychiatric illnesses and potential therapeutic approaches that could derive from work in this area.

Cells in the immune system generate chemicals that affect the way our bodies react to illnesses and influence how we feel and behave. These chemicals directly influence nerve cells, including those involved in the regulation of motivational, cognitive, and reward systems. For example, certain medical illnesses like coronary heart disease can influence the immune system. Immune system dysregulation associated with these medical disorders can also increase the risks for depressive disorders

In addition, specific types of immune cells called microglia reside in the brain. These cells monitor local environments in the brain and produce a variety of chemicals in response to stressors or cell damage. While neurons produce neurotransmitters that regulate communication between nerve cells, microglia produce immune-related chemicals that can directly alter the structural and functional connections between nerve cells.

These microglia help defend the brain from various types of insults. For instance, they produce immune-related substances to assist the brain in recovering from infections. In addition, neurons and microglia work together to keep the nervous system “in balance.” Neurotransmitters and immune-related chemicals apparently work side by side in maintaining normal function and structure. There are conditions, however, in which excessive or abnormal function of microglia (and the peripheral immune system) can result in brain dysfunction and contribute to the pathogenesis and symptoms of various illnesses. These illnesses include neurodegenerative disorders such as Alzheimer’s disease, as well as perhaps psychiatric disorders such as major depression, schizophrenia, and others. This is a rapidly evolving area of neuroscience with implications for the development of novel therapies.

There appear to be back and forth interactions between the nervous system and immune system. Each system can influence the other. Physiological and psychological stressors can impact both brain and immune function. Abnormal immune function can be associated with behavioral changes including depressive symptoms.

As more is learned about the relationship of depressive symptoms and immune system function, it is likely that certain types of depression may respond to treatments that target components of the immune system. It would not be at all surprising if terms such as “immunopsychopharmacology” become part of the language of psychiatry.

This column was written by Eugene Rubin MD, PhD and Charles Zorumski MD.

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