This is the final article in a four part series reviewing the influence of humor and laughter on physiological and psychological well‐being. This final article reviews the evidence for the effect of sense of humor, exposure to a humor stimulus and laughter on various immune system components, with a focus on the effects of laughter on natural killer cell cytotoxicity.

This research reports immune function and health outcomes in women with depression, as compared with a non-depressed control group. Using Psychoneuroimmunolgy theory and a descriptive comparison design, scores on the Beck Depression Inventory (BDI) were used to divide 40 non-hospitalized Caucasian women between the ages of 18 and 65 years into either the control or depression comparison group. Women with depression were found to report significantly more incidences of illness over the previous two months and they were found to have significantly more indicators of illness at the time of the exam as compared to the controls. However, contrary to what has been documented in some earlier studies of depression, women with depression were not found to have significantly different immune function measures as compared to the control group. There was also no significant correlation between scores on the BDI and natural killer cell cytotoxicity in this study. While these findings support a connection between depression and both increased self-report of illness and increased signs and symptoms of minor illness or inflammation on physical exam, this study was not able to document that these effects were related to decreased immune function, as measured by natural killer cell activity or white blood cell counts.

This article reviews the literature on psychoneuroimmunology and its theoretical underpinnings. It also reviews research related to the immune-system-link of stress, chronic stress, depression and health behaviors to breast cancer. Although the relationship between psychological characteristics and the immune system have been documented repeatedly, the significance of these effects has not been demonstrated in many studies empirically. The question whether the use of alternative therapies contributes to decreasing the progression of the diseased is often asked. Research in the area of psychoneuroimmunology can provide the answer to this question.