Final answer:
Cortisol is a hormone crucial for the stress response, aiding in energy provision and fight-or-flight reactions, but it can affect the immune system negatively if levels remain high over an extended period.
Step-by-step explanation:
Cortisol is a hormone important in the body's response to stress. When we perceive stress, the sympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis are activated. Cortisol affects virtually every organ in the body and helps provide energy in response to stressors, which is necessary for the fight-or-flight response. Prolonged exposure to elevated cortisol levels, however, can weaken the immune system and have negative health outcomes. Hunger and appetite can also be influenced by cortisol, though indirectly, because of its effects on blood sugar levels and metabolism.