Fevers

A fever is a rise in core body temperature above the normal level of about 37 °C, as a non-specific response to infection.Body temperature is controlled by the hypothalamus in the brain, which acts like a thermostat with a set point. When phagocytes such as macrophages meet a pathogen, they release cytokines (for example interleukin-1) into the blood. Some pathogens release toxins with the same effect. These fever-causing chemicals, called pyrogens, act on the hypothalamus and raise the set point, for example to 39 °C.The body now responds as if it were too cold: arterioles in the skin constrict (vasoconstriction), the person shivers and feels cold, and metabolic rate increases, so body temperature rises to the new set point. When the infection is controlled, pyrogen release falls and the set point returns to normal. The body then loses heat by sweating and vasodilation (the fever 'breaks').A fever helps the body fight infection:
• many pathogens reproduce fastest at about 37 °C, so a higher temperature slows their reproduction
• the immune response works faster at a slightly higher temperature, for example phagocytosis and lymphocyte division speed up
• some pathogens are damaged by high temperatures.However, a very high fever is dangerous. Above about 40 °C the body's own enzymes and other proteins begin to denature, and in young children fevers can trigger seizures (febrile convulsions). Drugs such as paracetamol lower the set point and reduce the fever.