Pattern Recognition Receptors - B. Catchpole, RVC 2008

The innate immune system recognises components of pathogens which are intrinsically foreign (i.e. not present on normal mammalian cells), such as:

    • Lipopolysaccharides of gram-negative bacteria
    • Peptidoglycans of gram-positive bacteria
    • Mannose sugars
    • D-isoform amino acids

These are known to the body as foreign as they are expressed as pathogen-associated molecular patterns (PAMPs)

  • PAMPs are recognised by pattern recognition receptors (PRRs) expressed on mammalian cells
    • Pattern recognition receptors are expressed on many different cell types, not just on phagocytes
    • Not all are expressed by all cells: different cell types express a different range of PRRs
    • PRRs are either intracellular, membrane-associated or soluble:
      • Recognition of pathogens via the cellular PRRs results in phagocytosis and inflammation
      • Recognition of pathogens via the humoral PRRs results in various killing mechanisms
  • Engagement of PRRs by PAMPs triggers:
    • Phagocytosis
    • The expression of cytokines, which brings about inflammation and other immune responses


Examples of Pattern Recognition Receptors
Receptor Location Ligands
TLR2 (Toll-like receptor) Cell Membrane Peptidoglycan of gram +ve bacteria
TLR3 Cell Membrane dsRNA of RNA viruses (e.g. avian influenza)
TLR4 Cell Membrane Lipoplysaccharide from gram-negative bacteria (e.g. E. coli, Salmonella)
TLR5 Cell Membrane Bacterial flagellin
TLR9 Cell Membrane Bacterial DNA (CpG DNA)
C-type lectins Soluble Carbohydrates, all bacteria, dead cells
fMLP Soluble Formyl peptides (i.e. all bacteria)
Complement receptors Soluble Fixed complement components (e.g. iC3b)
NOD2 Cytoplasm Peptidoglycan of gram +ve bacteria
dsRNA-dependent Protein Kinase Receptor Cytoplasm ds RNA of RNA viruses



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