Natural killer (NK) cells play a crucial role in the defense against tumors. Peripheral blood monocytes are routinely assayed for the activity of NK cells by using a 4hr-51Cr release assay, and subsets of the NK cells are determined with monoclonal antibodies directed against surface phenotypes of the NK cells. We reviewed associations of demographic characteristics and individual lifestyles with the NK cell activities and the NK cell subpopulation frequencies. The age-dependence of NK cell activities has been a matter of controversy, although there are a number of cross-sectional survey reports suggesting an increase in NK cell activities with aging. The elevation of an NK subset bearing a phenotype of CD16+(Leu11+) has been reported to coincide with an increase in NK cell activities. Males are said to have higher NK cell activities than females. Smoking tobacco seems to reduce the NK cell activities and cessation of the smoking habit clearly increases the activities. With respect to alcohol consumption, no clear relevance to the NK cytolytic capability has been reported. Polyunsaturated fatty acid in food lowers the NK cell activities, while acute and subacute physical exercises significantly enhance the NK cell activities and the NK cell numbers in the peripheral blood, and a physical training protocol over a several-week period heightens the NK cell activities even among non-trained individuals. The association of the number of daily sleeping hours with the activities has not yet been elucidated. Stressful life events, daily hassles, mood, and a pattern of behavioural characteristics or personalities have been described as altering the NK cell activities and subpopulation frequencies. A well-designed cohort study involving psychoneuroimmunulogical approaches is expected to clarify the effects of individual lifestyles on the NK cell activites and populations.
