Recognition of pathogens by cells of the innate immune system.

When we say that cells of the innate immune system can ‘recognise’ pathogens, what do we actually mean? By recognition we mean that molecules, or receptors, on cells of the innate immune system bind to other molecules that are present on pathogens but not present on our own cells. In this way the immune system can distinguish foreign objects and respond to a foreign pathogen but not respond against our own tissue or cells. It is a crucial feature of the immune system that it can target foreign objects. There are many receptors present on cells of the innate immune system and some.
An important family of receptors that are able to recognise a wide variety of pathogen-associated molecules has been identified. These are the Toll-like receptors (TLRs), which were named because of their structural relationship to Toll, a protein involved in development of Drosophila, the fruit fly. There are about 10 TLRs that have been identified in man and similar numbers in other species. The TLRs recognise a variety of microbial products such as doublestranded RNA (found only in viral infection), lipopolysaccharide (LPS) from bacterial cell walls, bacterial lipoproteins, unmethylated DNA (indicating it is of bacterial origin) and flagellin, a component of bacterial flagella. Different TLRs recognise different microbial products.