About Toll-like receptors

This post will define Toll-like receptors.  Toll-like receptors (TLRs) are an important family of immune system receptors that constitute the first line of defense system against microbes. Moreover they can recognize both invading pathogens and endogenous danger molecules released from dying cells and damaged tissues.   Especially they play a key role in linking innate and adaptive immunity.

Background of the innate immune system

The innate immune system provides an immediate defense mechanism upon damage or pathogen invasion.  This allows the adaptive immune system to initiate an antigen-specific response.  Short-term activation of the innate immune system is beneficial and provides cytoprotective mechanisms for tissue repair. They are a family of pattern recognition receptors (PRRs) that form the cornerstone of the innate immune response.

Toll-like receptors Classes

Toll-like receptors are a class of proteins that play a key role in the innate immune system. They are single-pass membrane-spanning receptors usually expressed on sentinel cells such as macrophages and dendritic cells.  They recognize structurally conserved molecules derived from microbes or viruses. Once these microbes have reached physical barriers such as the skin or intestinal tract mucosa, they are recognized by TLRs, which activate immune cell responses. The TLRs include TLR1, TLR2, TLR3, TLR4, TLR5, TLR6, TLR7, TLR8, TLR9, TLR10, TLR11, TLR12, and TLR13. Humans lack genes for TLR11, TLR12 and TLR13 and mice lack a functional gene for TLR10. The receptors TLR1, TLR2, TLR4, TLR5, TLR6, and TLR10 are located on the cell membrane, whereas TLR3, TLR7, TLR8, and TLR9 are located in intracellular vesicles (because they are sensors of nucleic acids).

TLRs received their name from their similarity to the protein coded by the toll gene in fruit flies.  This is what a TLR3 looks like in a protein diagram.

what are Toll-like receptors – protein structure

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