Appropriate and efficient recognition of pathogens by the immune system represents the key step in the initiation of the immune response. Recognition of pathogen associated molecular patterns is mediated by the set of germ line encoded pattern recognition receptors.
Toll-like receptors (TLRs) represent the most important member of pattern recognition receptors. It has been shown that defects in TLRs signalling pathways cause increased susceptibility to bacterial or viral infections.
Defects in signaling molecules downstream of TLRs recognizing bacteria have been described in patients with severe invasive pneumococcal infections. Defects in TLRs responsible for the induction of an anti-viral immune response have been detected in children with necrotizing herpetic encephalitis.