Cellular locations of pattern recognition molecules of the innate immune system

Some pattern recognition molecules of the Toll-like Receptor (TLR) family, such as TLRs 2, 4, and 5, are expressed on the cell surface, where they may bind extracellular pathogen-associated molecular patterns.  Other TLRs are expressed on endosomal membranes, such as TLRs 3, 7, 8, and 9, all of which can recognize nucleic acids of microbes that have been phagocytosed by cells.  Cells also contain cytoplasmic sensors of microbial infection, including the NOD-like Receptor (NLR) family of proteins, which recognize bacterial peptidoglycans, and a subset of Caspase Activation and Recruitment Domain (CARD) family of proteins, which bind viral RNA.