Fever: What's Happening Inside Your Body

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Inflammation is a local response of the body to injury. There are also systemic, or overall, responses; one of the most important is fever, an abnormally high body temperature above the normal range of 36.5–37.5 °C (97.7–99.5 °F) due to an increase in the temperature regulatory set-point. This increase in set-point triggers increased muscle tone and chills. Fever is the third component of the second line of defense and the most frequent cause of fever is infection from bacteria (and their toxins) or viruses.

Body temperature is controlled by a part of the brain called the hypothalamus. The hypothalamus is sometimes called the body's thermostat, and it is normally set at 37°C (98.6°F). It is believed that certain substances affect the hypothalamus by setting it at a higher temperature. Recall from Chapter 15 that when  phagocytes ingest gram-negative bacteria, the lipopolysaccharides (LPS) of the cell wall (endotoxins) are released, causing the phagocytes to release the cytokines interleukin-I (formerly called endogenous pyrogen), along with TNF-a. These cytokines cause the hypothalamus to release prostaglandins that reset the hypothalamic thermostat at a higher temperature, thereby causing fever.  

Assume that the body is invaded by pathogens and that the thermostat setting is increased to 39°C (I 02.2"F). To adjust to the new thermostat setting, the body responds by constricting blood vessels, increasing the rate of metabolism, and shivering, all of which raise body temperature. Even though body temperature is climbing higher than normal, the skin remains cold, and shivering occurs. This condition, called a chill, is a definite sign that body temperature is rising. When body temperature reaches the setting of the thermostat, the chill disappears. The body will continue to maintain its temperature at 39"C until the cytokines are eliminated. The thermostat is then reset to 37°e. As the infection subsides, heat-losing mechanisms such as vasodilation and sweating go into operation. The skin becomes warm, and the person begins to sweat. This phase of the fever, called the crisis, indicates that body temperature is falling.

Up to a certain point, fever is considered a defense against disease. Interleukin-I helps step up the production of T cells. High body temperature intensifies the effect of antiviral interferons and increases production of transferrins that decrease the iron available to microbes. Also, because the high temperature speeds up the body's reactions, it may help body tissues repair themselves more quickly. Among the complications of fever are tachycardia (rapid heart rate), which may compromise older persons with cardiopulmonary disease; increased metabolic rate, which may produce acidosis; dehydration; electrolyte imbalances; seizures in young children; and delirium and coma. As a rule, death results if body temperature rises above 44 to 46"C (112- 114"F).

Article sources:
Microbiology 10th Edition by Tortora
Wikipedia