The innate immune system uses a variety of pattern-recognition receptors (PRRs) that recognize conserved microbial structures or pathogen-associated molecular patterns, such as those that occur in the bacterial cell-wall components peptidoglycan and lipopolysaccharide. PPRs are classified as:1)Membrane bound PRRs: These are referred to as toll-like receptors (TLRs). Eleven such receptors have been identified in mammals. TLRs are transmembrane receptors which use adapter proteins like TRAF6 and Myd88 to initate NF-κB and other signaling pathways that induce pro-inflammatory cytokines. 2)Extracellular PRRs: These include the complement proteins, pentraxins and collectins. These molecules respond to bacterial and viral triggers through the process of opsonization, phagocytosis and apoptosis. 3)Cytoplasmic PRRs: These include the NODs, NALPs and PKRs. NODs for example react to peptidoglycans by activating NF-κ, therefore inducing the production of pro-inflammatory cytokines. PKRs react by inhibiting translation as well as inducing proinflammatory cytokines.