a, Chemical structure of LPS. LPS is composed of three modules: lipid A, a core oligosaccharide and a highly variable O-antigen constituted of repeating oligosaccharide units. The core is covalently linked to lipid A and can be further divided into inner and outer core. The inner core contains at least one residue of 3-deoxy-d-manno-oct-2 ulosonic acid (Kdo) linking to lipid A. b, Ra LPS is about 32 Å in height and 28 Å × 12 Å in the other two dimensions. The size of Ra LPS is based on the crystal structure of the TLR4/MD-2/Ra LPS complex (PDB ID 3FXI).