Why does your body temperature rise when you have a fever?

When you have an infection, your immune system releases signaling molecules called pyrogens, including cytokines like interleukin-1 and tumor necrosis factor. These molecules travel to the hypothalamus, the brain region that acts as your body's thermostat, and trigger the production of prostaglandin E2, which resets the hypothalamic set point to a higher temperature. Your body then works to reach this new higher temperature by constricting blood vessels to reduce heat loss and triggering shivering to generate heat through muscle activity. The elevated temperature is actually a defense mechanism, as it can slow bacterial and viral replication, enhance immune cell activity, and accelerate the biochemical reactions involved in fighting infection.