To regulate itself, the body often relies upon sensors that detect something amiss and then emit a signal that is amplified by the body so that a process can be set in motion to fix the issue that set the sensor off. One of the key signals the body relies upon are hormones, as small amounts of these molecules being released are often sufficient to change the internal state of the body drastically.

The hypothalamic-pituitary-adrenal (HPA) axis is the body's central stress response system. It has three main components: the hypothalamus and pituitary gland in the brain, and the adrenal glands on top of the kidneys. When you experience stress, the hypothalamus releases corticotropin-releasing hormone (CRH), which signals the pituitary gland to secrete adrenocorticotropic hormone (ACTH). ACTH then travels through the bloodstream to the adrenal glands, prompting them to release the corticosteroid cortisol (the body's primary stress hormone). Finally, once cortisol levels are high enough, they signal the brain to reduce CRH and ACTH production, creating a negative feedback loop that prevents over-activation of the stress response.

Cortisol, in turn, has a few key functions in the body:

Immune Modulation: Cortisol first enhances the immune system's immediate response to threats (protecting the body during stress), then limits excessive immune activity to prevent autoimmunity. It does this partly by inhibiting pro-inflammatory cytokines (e.g., IL-1, IL-6) and reducing T-cell activity. Over time, this shifts to immune suppression, making synthetic corticosteroids, a popular treatment for inflammation and autoimmunity.