The ability to have a body temperature that is independent of environment temperature is evolutionary acquisition. Thanks to him, organisms can control the intensity of metabolism to increase body temperature. Homoiothermic animals can maintain an optimum body temperature via the heat transfer into the environment. An optimal level of temperature is necessary for activity of most enzymes, the implementation of metabolic reactions in cells. The viscosity of blood and extracellular fluids, the surface tension of the surfactant, swelling colloids of the interstitial space depend on the temperature inside. The excitation of cells, muscle contraction, secretion, absorption, locomotor activity and phagocytic properties of neutrophils also depend on the temperature of the tissues. Fever is pathological process a temporary increasing of body temperature for homoiothermal animals and humans. The process starts due to cytokines exogenous and endogenous origin. Cytokines rebuild the system of thermoregulation. Fever is protective response to damaged but may be redundant. Tachycardia at a fever promotes arrhythmias and may be complicated by the development of acute heart failure. Critical temperature decrease with profuse sweating can cause collapse. Fever in pregnant women in the early stages of placentation provokes violations and defects of the neural tube of the embryo. Fever leads to accumulation of ketonic substances in the blood due to incomplete lipolysis. Ketoacidosis with fever in young patients is the cause of hyperthermic convulsions. The negative effects of fever forced to resort to antipyretic therapy and measures to increase heat dissipation.

temperature homeostasis, fever, cytokines, febrile convulsions .