A theory that demonstrates the close relationship between the nervous and immune systems, expressed in the direct impact of psychological stress on immune function. Chronic stress suppresses the immune response, increases susceptibility to somatic diseases, and even contributes to the exacerbation of chronic diseases such as cancer or autoimmune diseases. Thus, psychosomatic disorders are not just transient symptoms, but rather a manifestation of the disorder in a holistic system encompassing the brain, nervous, hormonal and immune systems, which explains their versatility, and in some cases, seriousness.