Innate Immunity : An Early Phase Of Defence Mechanism Against Intruding Microorganisms

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1.1 Innate immunity
The innate immunity provides an early phase of defence mechanism against intruding microorganisms, mediated by phagocytes such as macrophages and dendritic cells (DCs), having the ability to distinguish between self and non-self (pathogens). The innate immune system recognises microorganisms by germ-line encoded pattern recognition receptors (PRRs), expressed on all types of cells. The metabolic products generated by microbial pathogens rather than the host allow the immune system to differentiate between the self and non-self. The pattern recognition receptors function to identify the molecule elements on microbes, which are acknowledged as pathogen-associated molecular patterns (PAMPs). The elements of microbes that
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Microbial products initiate Toll-like receptors (TLRs), which in turn activate the signalling pathways, causing antimicrobial genes and inflammatory cytokine induction (Janeway and Medzhitov, 2002). The TLR subfamily TLR1, TLR2, and TLR6 recognise lipids where as TLR7, TLR8, TLR9 recognise nucleic acids. TLRs are expressed on DCs, macrophages, T cells, B cells, epithelial cells and fibroblast cells, to modulate a response to pathogens and cytokines. The TIR domain of TLR involved in the interaction of TLRs and adaptor proteins such as MyD88, TRIF, TRAM, and TIRAP/MAL, regulate TLR signalling. This leads to the downstream signalling cascade of the adaptor molecules (Figure 2), stimulating proinflammatory cytokines, chemokine’s and types I and type III interferons. The adaptive immune response is also activated by antigen presenting cells (APCs), which present antigens to naïve CD4+ T cells. The naïve CD4+ T cells can be differentiated by DCs, into T helper 1 (Th1) cells, which generate interferon-y (IFN-Y) for the elimination of infected cells (Akira, Uematsu and Takeuchi, 2006) .

Figure 1: Toll like receptor and NOD like receptor family of ligands, involved in innate immunity. 11 TLRs have been identified in humans and 13 in mice. The innate immune system can differentiate different types of pathogens to induce the correct flow of effector adaptive response, as a result of TLRs that
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