Chemical structure of lipopolysaccharides and endotoxin immunity.

Lipopolysaccharides (endotoxins) of gram-negative bacteria consist of 2 components with distinct physico-chemical character: a heteropolysaccharide and a covalently linked lipid, termed lipid A. The chemical structure of lipid A, which represents the toxic center of lipopolysaccharides, is discussed. Evidence is presented that lipid A antiserum suppresses the pyrogenic effect of lipid A and lipopolysaccharides in rabbits. The protective power of lipid A antiserum, however, is only expressed in animals which have been pretreated with lipid A or lipopolysaccharide indicating that other than humoral factors, perhaps cellular, also participate in endotoxin fever (cross) immunity. The fever resistance mediated by lipid A antiserum seems to be endotoxin-specific with regard to both the preparative and the challenging injection. Lipid A antiserum therefore may serve as a tool to discriminate between fever caused by endotoxins and that induced by other pyrogens.