Toll Like Receptors in Dual Role: Good Cop and Bad Cop

Abstract

Every living organism tends to protect itself from harmful effects of pathogens or molecules of pathogenic origin that can disturb its well-being state. The first line of defence that comes into action upon encounter with the pathogen is referred to as innate immune defence mechanism. It had been a matter of great inquisitiveness how innate immune defence mechanism is able to render the body protected against such a diverse variety of pathogens. But with the discovery of germ line encoded pattern recognition receptors (PRRs) that can sense the pathogen associated molecular patterns (PAMPs), it is to an extent possible to answer the query, how innate immune system copes to recognise such a wide variety of micro-organisms and harmful microbial elements. PAMPs are usually of pathogenic origin and absent from the cells of host origin. PRRs can be transmembrane receptors like Toll like receptors (TLRs) (Beutler & Rietschel, 2003; Janeway & Medzhitov, 2002), C-type lectin receptors (CLRs) or these can be cytosolic receptors like Nod like receptors (NLRs) and Rig like helicases (RLRs). Every PRR is capable of recognising specific conserved molecular patterns on the micro-organism and later can start a downstream signalling process upon proper interaction of PAMP and PRR that leads to synthesis of effector molecules like antimicrobial peptides and pro-inflammatory cytokines that prevent the body from otherwise harmful microbes.