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Endotoxin is a biologically active substance produced by bacteria and refers to the lipopolysaccharide (LPS) complex associated with the outer membrane of Gram-negative bacteria. Endotoxin is unique in that its recognized mode of action does not result from direct damage to host cells and tissues. Rather, endotoxin stimulates cells of the immune system, particularly macrophages, and of the vascular system, to become activated and to synthesize and secrete a variety of effector molecules that cause an inflammatory response at the site of bacterial invasion. Endotoxins cause white blood cells to produce interleukin-1, a hormone responsible for fever malaise, headache, muscle aches, and other nonspecific consequences of infection. It is estimated that as many as 50,000 deaths annually occur in the United States as the result of endotoxin-induced shock.