Fig. 5: Pattern-Recognition Receptors(PRRs) on and in Defense Cells

To recognize PAMPs such as those listed above, various body cells have a variety of corresponding receptors called pattern-recognition receptors or PRRs capable of binding specifically to conserved portions of these molecules. Cells that typically have pattern recognition receptors include macrophages, dendritic cells, endothelial cells, mucosal epithelial cells, and lymphocytes. Many pattern-recognition receptors are located on the surface of these cells where they can interact with PAMPs on the surface of microbes. Others PRRs are found within the phagolysosomes of phagocytes where they can interact with PAMPs located within microbes that have been phagocytosed. Some PRRs are found in the cytosol of the cell.

on the surface:
TLR-2 - recognizes peptidoglycan, bacterial lipoproteins, lipoteichoic acid, and porins
TLR-4 - recognizes lipopolysaccharide (LPS) from gram-negative cell wall, fungal mannans, viral envelope proteins, parasitic phospholipids, heat-shock proteins
TLR-5 - recognizes bacterial flagellin

within endosomes (phagolysosomes):
TLR-3 - recognizes viral double-stranded DNA
TLR-8 - recognizes viral single-stranded RNA
TLR-9 - recognizes viral and bacterial unmethylated CpG sequences

in the cytoplasm:
NOD-2 - recognizes muramyl dipeptide from bacterial peptidoglycan
- recognizes viral RNA

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