ABSTRACT
Introduction Mammals have two distinct immune systems; innate and acquired. One of the hallmarks of the
acquired immune system is its almost infinite variety of receptors for antigen, which arise during genetic rearrangement and somatic hypermutation. Moreover, the acquired immune system has memory characteristics, which enable specific responses to secondary infection and form the principle underlying vaccination. In contrast, the innate immune system was previously thought of as primitive, unsophisticated, and only helpful up until the acquired immune system starts to work. However, the discovery ofToll as a receptor recognizing invading pathogens in the fruit fly Drosophila melanogaster} followed by cloning of mammalian Toll-like receptor,2 revealed that the innate immune system is able to recognize a variety of molecules derived from pathogens, and even to control acquired immunity. Ten and 13 Toll-like receptors (TLRs) have so far been identified in human and mouse, respectively.3 These germ line-encoded receptors can sense a broad range of compounds called pathogen-associated molecular patterns (PAMPs), which include proteins, bacterial cell wall components such as sugars and lipids and nucleic acids. After recognition, TLRs induce intracellular signaling cascades that lead to pleiotropic responses, including the production of proinflammatory cytokines, and consequendy contribute to the activation of the acquired immune response.
