Physical and psychosocial stressors have been shown to compromise immune function (Ader et al., 1991; Kielcolt-Glaser and Glaser, 1995). The immune suppressive effects of stress may be more pronounced in individuals that already have limited immune competence, such as infants, individuals with a predisposition to autoimmune disease, and the elderly (Kielcolt-Glaser and Glaser, 1995). An individual's response to a stressor is manifested in physiological, hormonal, behavioral, and immunological changes. These stress-induced responses are initiated by the hypothalamus and translated into action by the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic nervous system. Products from these two systems (e.g., corticoid hormones and catecholamines) can directly modulate the activity of various immune effector cells (Ader et al., 1991). © 2006 Springer Science+Business Media, LLC.