Psychoneuroimmunology explores the connection between the mind, body and immune system. There have been several studies that support the idea that the connection between the brain and immune system affect the quality of life, and possibly the morbidity and mortality rates of cancer patients.
According to one study, considerable work over the past decade has shown how psychological processes can impact pathways implicated in cancer progression. Furthermore, immune system dysregulation may have major implications for fatigue and depressive symptoms among cancer survivors. 
In the article, Psychoneuroimmunology and cancer: fact or fiction? by Kiecolt-Glaser JK1 and Glaser R., they state:
“There is substantial evidence from both healthy populations as well as individuals with cancer linking psychological stress with immune downregulation. This discussion highlights natural killer (NK) cells, because of the role that they may play in malignant disease. In addition, distress or depression is also associated with two important processes for carcinogenesis: poorer repair of damaged DNA, and alterations in apoptosis. Conversely, the possibility that psychological interventions may enhance immune function and survival among cancer patients clearly merits further exploration, as does the evidence suggesting that social support may be a key psychological mediator. These studies and others suggest that psychological or behavioral factors may influence the incidence or progression of cancer through psychosocial influences on immune function and other physiological pathways.” 
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Psychoneuroimmunology (Mind-Body Medicine)
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Psychological stress has been found to be the main contributor in affecting immune response. This is explored in an article in the Journal of the National Cancer Institute, Psychologic Stress, Immunity, and Cancer by Sheldon Cohen and Bruce S. Rabin. They explore five areas that stress can affect cancer patients:
- Psychologic stress can alter immune function.
- The immune system plays a role in regulating tumor growth.
- Immune changes under stress are of the type that would influence tumor growth and metastasis.
- Immune changes under stress are of the magnitude that would influence tumor growth and metastasis.
- Stress-reduction interventions will influence the progression of the disease.