Final answer:
Macrophages release heat cytokines known as pyrogens, which cause fevers by resetting the hypothalamus to raise the body's temperature, thereby serving as a defense mechanism.
Step-by-step explanation:
Macrophages produce heat cytokines called pyrogens, which reset the body's thermostat to a higher temperature, resulting in fever. When bacteria are destroyed by leukocytes, these endogenous pyrogens are released into the blood. The pyrogens circulate to the hypothalamus, which is responsible for maintaining the body's temperature set point. Upon reaching the hypothalamus, pyrogens induce a reset of the body's 'thermostat' to a higher temperature, leading to fever.
This increase in body temperature has multiple effects: it conserves iron, limiting a nutrient needed by bacteria; it increases the activity of enzymes and protective cells in the host while inhibiting those of the invading microorganisms; and the elevated heat may directly kill pathogens. It is now recognized that a fever is not just a symptom of infection but rather a normal defense mechanism of the body.