A three stage model about stress, devoloped by Hans Selye. The first stage is Alarm Reaction. In this stage the body gets geared up to either fight or flee the current stressor. The HPA axis and sympathetic nervous system are activated by the body. This leads to increased mental activity and physiological arousal. The second stage is resistance also called adaptation. In this stage, defense and adaptation are sustained and optimized, until homeostasis can be reached again. The body continues to experience physiological arousal and to expend energy as it continues to cope with the stressor. In this stage the immune system begins to weaken. The third and final stage is exhaustion. In this stage the body gives up for physical energy is depleted, the immune system is is suppressed, thinking begins to fail, attention is impaired, adaptive responses cease, and this stage can be followed by illness, or sometimes even death.
source: Abnormal Psychology
by Martin E. P. Seligman, Elaine F. Walker and David L. Rosenhan