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The Immune Response Innate Immunity As they have co-evolved with microbes, higher organisms have developed mechanisms for recognizing and responding to microorganisms. Many of these mechanisms, referred to together as innate immunity, are evolutionarily ancient, having been conserved from insects to humans. In general, innate immune mechanisms exploit molecular patterns found specifically in pathogenic microorganisms. These "pathogen signatures" are recognized by host molecules that either directly interfere with the pathogen or initiate a response that does so. Innate immunity serves to protect the host without prior exposure to an infectious agent—i.e., before specific or adaptive immunity has had a chance. | Chapter 113. Introduction to Infectious Diseases Host-Pathogen Interactions Part 4 The Immune Response Innate Immunity As they have co-evolved with microbes higher organisms have developed mechanisms for recognizing and responding to microorganisms. Many of these mechanisms referred to together as innate immunity are evolutionarily ancient having been conserved from insects to humans. In general innate immune mechanisms exploit molecular patterns found specifically in pathogenic microorganisms. These pathogen signatures are recognized by host molecules that either directly interfere with the pathogen or initiate a response that does so. Innate immunity serves to protect the host without prior exposure to an infectious agent i.e. before specific or adaptive immunity has had a chance to develop. Innate immunity also functions as a warning system that activates components of adaptive immunity early in the course of infection. Toll-like receptors TLRs are instructive in illustrating how organisms are detected and send signals to the immune system. There are at least 11 TLRs each specific to different biologic classes of molecules. For example even minuscule amounts of lipopolysaccharide LPS a molecule found uniquely in gram-negative bacteria are detected by LPS-binding protein CD14 and TLR4 see Fig. 114-3 . The interaction of LPS with these components of the innate immune system prompts macrophages via the transcriptional activator nuclear factor kB NF-kB to produce cytokines that lead to inflammation and enzymes that enhance the clearance of microbes. These initial responses serve not only to limit infection but also to initiate specific or adaptive immune responses. Adaptive Immunity Once in contact with the host immune system the microorganism faces the host s tightly integrated cellular and humoral immune responses. Cellular immunity Chap. 308 comprising T lymphocytes macrophages and natural killer cells primarily recognizes and combats pathogens that proliferate .
