The interaction of bacteria and other pathogenic microorganisms with the mammalian immune system has been the subject of world-wide investigations since more than a century now. The studies carried out in the group of Marina A. Freudenberg at the MPI for Immune Biology are concerned mainly with the interaction of lipopolysaccharide (endotoxin, LPS) with the innate immune system. LPS is a highly toxic component present in the outer cell-wall of Gram-negative bacteria, inducing in animals and humans a large spectrum of pathophysiological activities that can lead to shock and death. On the other hand LPS is a powerful activator of the innate immune system and plays a primary role in the early recognition of bacterial infections and in the stimulation of antibacterial defense. The positive and negative consequences of LPS/host interaction during bacterial infections, i.e. the induction of an early resistance to infection, the development of pathophysiological effects, as well as their underlying mechanisms are investigated by the group in different mouse-models. The objective of these studies is an improved diagnosis, prevention and treatment of infection and are therefore important both from the scientific and clinical point of view.