Lipopolysaccharides are biological molecules made up of a complex of fats and sugars. There are commonly found in the cell walls of gram-positive bacteria, but not the cell walls of human cells. As such, in humans and other animals, the mere presence of these molecules will trigger a response by the immune system. Several cells in the immune system respond directly to lipopolysaccharide molecules and a high concentration of such molecules will trigger septic shock in humans.
It's believed many autoimmune conditions are caused by other molecules that mimic lipopolysaccharides.