In order to illustrate how stress and hives are related, it is necessary to have an understanding of the body's general response to stress. Webster's Dictionary defines stress as, "any stimulus that disturbs or interferes with the normal physiological equilibrium of an organism." According to Stanford professor Robert M. Sapolsky, "Stress may be the thing that takes a part of our body that was marginally damaged by exposure to God-knows-what sort of toxins and pushes it into overt disease." (6) There is little doubt in the medical community that the mind and body are inextricably linked by a delicate biochemical equilibrium. Stress attacks homeostasis, causing the body to instinctually react to the potential danger by increasing the release of the hormones adrenaline and cortisol by the sympathetic nervous system. (7) This reaction is referred to as "fight or flight" or an "adrenaline rush" in common terminology. 

In very simple terms, cortisol helps put more glucose in the blood and provides protection against inflammation or tissue injury. Adrenaline quickly turns the glucose into extra energy while activating the body's immune response. This response causes aggregation of platelets, an increase in heart rate and blood pressure, rapid breathing, perspiration, and such. This activation basically allows the body to fight a perceived threat or run away from it. Ideally, the exposure to the stressor is eliminated and the endocrine and immune systems quickly restore hormonal balance and calm the brain down. Stress can serve as a catalyst for physical changes, either strengthening the body's resilience or weakening its resistance to illness. Paradoxically, the added stress created when one is afflicted with disease perpetuates the illness, further weakening the immune system. (8) 

The stress response becomes unhealthy when the body is not given an opportunity to shut down the stress reaction system long enough to restore the proper hormonal balance. (9) The length of down-time required for this process individually varies with stress intensity and how one chooses to react to the stress. (10) It is found that prolonged day-to-day stress causes a weakening of the response system, making the body unable to internally recover following a stressor. Stress also causes a hormonal imbalance with excess cortisol and adrenaline in the system which, in turn, can cause damage to the hippocampus; site of emotion, memory, and learning in the brain, and suppress the immune system. (5) Chronic or persistent illnesses are signals from the body that it has a hormonal imbalance from either exposure to the stress response for too long or the methods being used for coping with stress are inefficient. (11) The wide-spread damage to the body caused by stress is so great that a new scientific field, psychoneuroimmunology, has been created to study and treat the diseases and conditions associated with the link between the brain and the immune system. (6)