Toll‐like receptors: molecular mechanisms of the mammalian immune response

The unrelenting bombardment by microbial pathogens from the environment and their increasing resistance to medical treatments are a constant threat to the survival of all organisms on earth. Microbes are covered by molecular patterns that are common among a broad range of pathogens. These include the lipopolysaccharides (LPS) of Gram-negative bacteria, lipoteichoic acids of Gram-positive bacteria, lipoproteins of bacteria and parasites, glycolipids of mycobacteria, mannans of yeast and double-stranded RNAs of viruses.1,2 Recognition of and responses to these molecules are controlled by a wide variety of cellular receptors. The best characterized receptors are the T-cell receptor and B-cell antibody receptor of the adaptive immune response, the specificity of which is randomly generated and clonally selected during the development of T and B lymphocytes.3 Unlike the receptors of the adaptive immune system, the pattern recognition receptors of the innate immune system have predetermined specificity generated early on in evolution and play an essential role in the determination of self versus non-self during the initial rapid responses to infection.1,2 As described in this review, a family of cell surface receptors, termed Toll-like receptors, are emerging as key regulators of host responses to infection.