The field of psychoneuroimmunology has witnessed an explosion of empirical findings during the last two decades. Research has documented the mechanisms through which stressful emotions alter white blood cell function. Stress diminishes white blood cell response to viral infected cells and to cancer cells. Moreover, vaccination is less effective in those who are stressed and wounds heal less readily in those who are stressed. While stress decreases the activity of some white blood cells, stress does not compromise the function of all types of white blood cells. Indeed, some types of autoimmune disease, which involve particular subsets of white blood cells, are exacerbated by stress. The literature documents the efficacy of talk-therapy interventions in altering immune system parameters and enhancing the body's ability to combat disease. The literature also documents the impact of the chronic stress of poverty on immune system function.