TLR4 and infection: Other lipid antigens that activate NKT cells include glycosphingolipid isoglobotrihexosylceramide (iGb3), an endogenous antigen synthesized in the endoplasmic reticulum (ER) and Golgicomplex (34, 35), and bacteria-derived lipopolysaccharide (LPS), a ligand for Toll-like receptor 4 (TLR4) expressed on NKT cells, which corresponds with the roles that NKT cells play during exogenous infection (8, 36, 37).