Immunogenic properties of lipid A.

Lipid A, the endotoxic principle of gram-negative bacterial lipopolysaccharides, when suitably exposed on the surface of bacterial cells, becomes immunogenic, eliciting the formation of antibodies specific to lipid A. Antibodies to lipid A occur naturally in the serum of normal humans and many animal species; the two exceptions found so far are mice (all strains tested) and guinea pigs. Antibodies to lipid A occur more frequently in patients with infections caused by gram-negative bacteria, such as urinary tract infections. Antibodies to lipid A interacted with free lipid A but not with lipid A as present in the intact lipopolysaccharide molecule. Antibodies to lipid A were induced in rabbits, but not in mice, following a single injection of immunogenic lipid A. Because of similarities between the lipid A of many gram-negative bacteria, antibodies to lipid A showed wide cross-reactions. With regard to biologic activity, antibodies to lipid A have been variously shown to be protective, damaging, or without effect.