Lipopolysaccharide

Lipopolysaccharides (LPS), also known as lipoglycans, are large molecules made of a lipid and a polysaccharide joined by a covalent bond.
They occur in the outer membrane of Gram-negative bacteria, and act as triggers for the innate immune system. Their presence is sensed by toll-like receptors on dendritic cells. This causes a strong immune response in animals.