Innate immunity comprises both soluble (eg complement, lysozyme) and cellular effectors (eg natural killer [NK] cells, macrophages and dendritic cells [DCs]). The innate and adaptive immune systems are principally bridged by the action of specialised APCs, which translate and transfer information from the body tissues and innate immune system to the adaptive immune system, selleck allowing a systemic response to a localised threat. The innate immune system therefore drives and shapes the development of adaptive immune responses via chemical and

molecular signals delivered by APCs to induce the most appropriate type of adaptive response. The adaptive immune system forms the second, antigen-specific line of defence, which is activated and expanded in response to these signals. Cells of the innate immune system are produced in the bone marrow and then migrate to different anatomical locations. The innate immune cell repertoire includes tissue-resident cells such as macrophages and immature DCs, and cells which circulate via

blood and the lymphatic system, such as monocytes, neutrophils, eosinophils, NK cells and innate T cells. Non-immune system cells at vulnerable locations, this website including keratinocytes and other epithelial and mucus-producing cells, fibroblasts and endothelial cells, can also exhibit innate defensive behaviours. Invading pathogens are detected by the innate immune system through molecular-sensing surveillance mechanisms. These mechanisms include detection of pathogens via pattern recognition receptors

(PRRs), expressed by cells of the innate immune system, which can be secreted, or expressed on the cell surface, or are present in intracellular compartments (eg DNA/RNA sensors). Examples of PRRs are the transmembrane Toll-like receptors (TLRs) and Table 2.1 lists the qualities of several TLRs. The model system in Figure 2.4 illustrates the location of the main human PRRs, and highlights the signalling pathways of several mammalian Liothyronine Sodium TLRs. The key feature of cells of the innate immune system is their ability to directly recognise different classes of pathogens – eg viruses and bacteria – by PRRs. These receptors are able to bind to molecules (such as bacterial membrane components) that are shared by several pathogens (eg all Gram-negative bacteria express lipopolysaccharide [LPS]), enabling the innate immune system to sense the occurrence of an infectious event. Recently, DCs and macrophages have been shown to react to signals released by damaged cells, indicating that the innate immune system can react to both the presence of infectious microbes (via pathogen-associated molecular patterns [PAMPs]) and to the consequences of an infectious event. Epithelial cells, fibroblasts and vascular endothelial cells are also able to recognise PAMPs, and signal to innate immune cells when infected, stressed or damaged.