The Stress Mechanism There are two major types of stress that a body can undergo: Acute stress of a fight/flight situation. This type of stress also enhances the immune system in the short term. For the brain, acute stress enhances the memory of events that are potentially threatening to the organism so that if the situation is encountered in the future, action and reaction can be immediate or "reflexive". Chronic stress is a longer term stress usually placed on the body's immune system by invasive organisms such as bacteria and viruses. Chronic stress causes adaptions in the brain, in which local neurotransmitters as well as systemic hormones interact to produce structural as well as functional changes to combat the infectious stresses. The following explanation covers mainly the Acute Stress body reaction mechanisms. All glands in the endocrine systems "dance" with each other in a feedback loop. The feedback mechanism in this page describes the dance between the Hypothalamus, Pituitary, and Adrenal glands (The HPA system) based on stressors or stresses to the body and the body's subsequesnt reaction to it. Organization of the pituitary-adrenal-axis (HPA): Stressful environmental stimuli trigger the release of corticotrophin releasing factor (CRF) from nerve cells in the the hypothalamus into a blood system that travels to the anterior pituitary. In response to CRF, the anterior pituitary releases adrenocorticotrophic hormone (ACTH) into the blood system. ACTH is transported to the adrenal gland through the blood. ACTH stimulates the production of cortisol in the adrenal cortex. Cortisol is released into the blood stream The feedback system. The hypothalmic-pituitary-adrenal-axis is a complex feedback mechanism. Cortisol feeds back to the hypothalamus to control release of CRF. High levels of cortisol in the blood inhibit the release of CRF by the hypothalamus, consequently less ACTH is released by the anterior pituitary, which in turn, causes less cortisol to circulate in the blood stream. Stress can have immunosuppressive effects and the hypothalamus-pituitary-adrenal axis (HPA) is a key factor in the neural modulation of the immune system. Stress and the Immune System The hormones secreted by the hypothalamus and pituitary gland affect the actions of numerous endocrine organs, such as the sex organs, the adrenal glands, and thryroid gland. A number of hypothalamic-influenced pituitary hormones can influence immune functioning, such as HGH, prolactin, TSH, FSH, and LH.  Fight/Flight Mechanism   Pituitary secretions within the HPA system act on the adrenal glands in both the adrenal medulla and the adrenal cortex, to produce changes similar to the "fight or flight" response mediated by the sympathetic nervous system.   Bodily reactions to this hormone include: A "pumped" feeling with excess energy Increased heartbeat (thumping) deep and rapid breathing - in extreme cases, hyperventilation Increased and focused attention Increased pain thresholds Diminished hunger and Sexual appetite. The immune system organs, such as the bone marrow, the thymus gland, and the spleen, are integrated parts of this complex system. They participate in this system through direct stimulation by the sympathetic nerve fibers and receptors on immune cells for HPA secretions. ©1998-2009 Medical Central Online™, All Rights Reserved.