A growing body of evidence shows that the physiological systems associated with the response to stress are potent contributors to illness. The response to stressful challenges helps to maintain constant and appropriate internal conditions, called homeostasis. The stress response involves reactions to emergencies and a rapid and pervasive adjustment of internal states to prepare the organism for fight or flight, but long-term behavioral, physiological, and psychological factors contribute. The cumulative, converging effects of these various factors culminate in patterns shown in the physiological mediators of the stress response including the failure to shut them off when they are not needed. The cost to the body produced by overactive mediators is called allostatic load. Allostasis is the process of adaptation and connotes the maintenance of stability (or homeostasis) through change. Allostasis thus describes a process of adaptation to challenge; allostatic load is the wear and tear on the body as a result of repeated allostatic responses.
Allostatic load is more than chronic stress. It can also reflect a genetically or developmentally induced failure to cope efficiently with the normal challenges of daily life. Developmental influences are implicated in influencing individual susceptibility to stress-related disorders. Changes in balance among neurotransmitters in the brain from the time of early development through adulthood to old age can influence behavioral responses to potentially stressful situations, can alter the interpretation of stimuli, and might be associated with anxiety and depression. Research with laboratory animals indicates that early-life experiences strongly influence lifetime allostatic load. For example, in laboratory animals poor maternal care is associated with increased behavioral and stress hormone reactivity in adult life.
The immune system is highly integrated with other physiological systems. It is sensitive to virtually every hormone, and sympathetic, parasympathetic, and sensory nerves innervate the organs of the immune system. The nervous, endocrine, and immune systems communicate bidirectionally through common hormones, neuropeptides, and cytokines. Stress-induced activation of neuroendocrine pathways has been shown to modulate various physiological systems, including the immune system. Stress-induced modulation of the immune system has been linked to the expression of inflammatory, infectious, and autoimmune diseases.
Several psychological factors—including hostility, anger, depression, and vital exhaustion—have been associated with susceptibility to diseases