Toll-like receptors

Toll-like receptors (TLRs) are a family of immune receptors that are used by mammals for the innate immune recognition of microbes. Their name derives from sequence homology to the Drosophila melanogaster gene Toll. In flies, Toll was first identified as a gene important in embryogenesis in establishing the dorsal-ventral axis. Later, Toll was found to have a role in the fly's immunity to funguses. This last fact was discovered at about the time Toll-like receptors were identified in mammals.

There are 13 Toll-like receptors that have been identified, but some are not present in humans, and some are not present in mice. Each Toll-like receptor works as either a homodimer or heterodimer in the recognition of a specific or set of specific molecular determinants present on microbes. Because the specificity of Toll-like receptors (and other innate immune receptors), the receptors must recognize determinants that are expressed and are not subject to mutation in the microbes, while not being present on the host. Thus, TLRs recognize molecules or parts of molecules that are extremely well conserved, such as lipopolysaccharide, lipopeptides, lipoarabinomannan, certain RNAs and DNAs, and flagellin.

The discovery of the Toll-like receptors finally identified the innate immune receptors that were responsible for many of the innate immune functions that had been studied for many years. Interestingly, TLRs seem only to be involved in the cytokine production and cellular activation in response to microbes, and do not play a significant role in the adhesion and phagocytosis of microbes.