A fever is a common physiological response, indicating the body’s defense against perceived threats. It represents a temporary elevation in core temperature, a deliberate adjustment supporting the immune system’s efforts to counteract infection or inflammation. This elevated temperature is not an illness itself, but a sign the body is working to restore balance.
The Body’s Internal Thermostat
The human body maintains a stable internal temperature, typically 97°F to 99°F (36.1°C to 37.2°C). This precise temperature regulation is managed by a complex brain region called the hypothalamus, often referred to as the body’s internal thermostat. The hypothalamus continuously monitors the body’s current temperature and compares it to a predetermined set point.
When the body’s temperature deviates from this set point, the hypothalamus initiates mechanisms to bring it back into the normal range. For instance, if the body gets too warm, it might trigger sweating, where evaporation of sweat helps cool the body. Conversely, if the body becomes too cold, the hypothalamus can induce shivering to generate heat or cause blood vessels near the skin surface to constrict, reducing heat loss. This constant fine-tuning ensures the body’s metabolic processes function optimally.
The Immune System’s Signal
A fever occurs when the immune system detects foreign invaders, like bacteria or viruses, or responds to inflammation. These invaders, or the body’s own immune cells, release chemical substances known as pyrogens. Pyrogens can originate from microbes or be produced by the body’s immune cells.
These pyrogens travel through the bloodstream to the hypothalamus, signaling it to raise the body’s temperature set point. Prostaglandin E2 (PGE2) is a key mediator, produced in the hypothalamus, that effectively resets the thermostat to a higher temperature. This elevated temperature is a deliberate defensive strategy, as many pathogens struggle to thrive and reproduce in higher heat environments. It also enhances the activity of immune cells, making the body a less hospitable place for invaders and boosting the overall immune response.
How the Body Heats Up
Once the hypothalamus resets the temperature set point to a higher level, the body initiates physiological responses to reach this new, elevated temperature. One mechanism involves vasoconstriction, where blood vessels beneath the skin narrow. This action redirects blood flow away from the body’s surface and towards the core, reducing heat loss and conserving warmth internally.
Alongside heat conservation, the body actively generates more heat. Shivering, involving rapid, involuntary muscle contractions, is a significant way the body produces warmth. These muscle movements consume energy and release heat as a byproduct. Individuals experiencing a rising fever might also instinctively seek warmer environments or bundle up, behavioral adjustments that further aid in reaching the new, higher temperature set point. These mechanisms continue until the body’s core temperature matches the hypothalamus’s new, elevated setting.