Fever and Related Host Defense Responses

Summary

This review focuses on the mechanisms underlying the induction of febrile, acute-phase, and ACTH responses by pyrogens. Systemic administration of a large dose of bacterial endotoxin (lipopolysaccharide, LPS) produces a biphasic fever. The first phase is mediated by endogenous pyrogen (EP) acting via structures outside the blood-brain barrier, while the second phase is mediated by EP acting via structures within the blood-brain barrier. Brain endothelial cells and the vagus nerves, which are both located outside the blood-brain barrier, are responsible for the induction of the first phase. The organum vasculosum laminae terminalis (OVLT) may be a site via which circulating EP enters the brain, or it may be the site at which EP is produced before its diffusion into the brain. Either way, intracerebral EP elicits the second phase. Similarly, two separate EP-related mechanisms are involved in the acute-phase responses induced by LPS (e.g., changes in plasma zinc, copper, fibrinogen, and leukocyte count), one inside and one outside the blood-brain barrier. Prostaglandin E, which stimulates the secretion of corticotropin-releasing factor, contributes to the increase in the plasma concentration of ACTH induced by both immunological (pyrogens) and non-immunological stress. The febrile, acute-phase, and ACTH responses participate in the host’s defense reaction against such pathogenic stimuli as bacterial infections, exercise, and psychological stress.