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Fever appears to have evolved in vertebrate hosts as an adaptive mechanism for controlling infection. This phenomenon is produced by certain exogenous (largely microbial) stimuli that activate bone-marrow-derived phagocytes to release a fever-inducing hormone (endogenous pyrogen). Endogenous pyrogen, in turn, circulates to the thermoregulatory center of the brain (preoptic area of the anterior hypothalamus) where it causes an elevation in the “set-point” for normal body temperature. Warm blooded animals produce fever by increasing heat production (through shivering) or reducing heat loss (by peripheral vasoconstriction), whereas cold blooded animals do so only by behavioral mechanisms (seeking a warmer environment). This paper discusses current concepts that involve the mechanism of endogenous pyrogen production, the role of central transmittors, and the probable function of fever in combating disease.