Psychoneuroimmunology is the study of the interactions among behavior, neural, and endocrine functions and the immune system. The purpose of this review is to briefly summarize the evidence concerning interactions among behavior, the neuroendocrine system, and the immune system, and to show how this evidence relates to critical care patients. It has been shown that the immune function of many patients in the intensive care unit is suppressed as a result of trauma, sepsis, or profound physiologic and psychological stress. Three of the most common stressors among patients in the intensive care unit are pain, sleep deprivation, and fear or anxiety. Findings have shown each of these stressors to be associated with decreased immune functioning. Nurses have an important responsibility to protect their patients from infection and promote their ability to heal. Several actions are suggested that can help the nurse achieve these goals. It is hoped that nurses would keep these interactions in mind while caring for their patients in the intensive care unit.