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An overview of the innate immune system

Author
Richard B Johnston, Jr, MD
Section Editor
E Richard Stiehm, MD
Deputy Editor
Anna M Feldweg, MD

INTRODUCTION

Humans live in an environment teeming with microorganisms and could not exist as a species without highly effective mechanisms of host defense. The innate immune system constitutes the first-line barrier, the rapid-response mechanism to prevent microbial invasion. Its components are inherited from parent to child and directed against molecules expressed only by microorganisms. These host defense components are evolutionarily ancient, found in all multicellular organisms, and expressed in humans as conserved elements (homologs) shared with other vertebrates and some insects and plants [1-3].

This topic will review the cells, proteins, and receptors that comprise the innate immune system, the functional differences between innate and acquired immune responses, and the mechanisms by which the two systems interact. Disorders of innate immunity and more specific topics on individual types of cells and receptors are presented separately. (See "Toll-like receptors: Roles in disease and therapy" and "Complement pathways" and "Mannose-binding lectin".)

INNATE VERSUS ADAPTIVE IMMUNITY

"Innate" immunity refers to immune responses that are present from birth and not learned, adapted, or permanently heightened as a result of exposure to microorganisms, in contrast to the responses of T and B lymphocytes in the adaptive immune system [4]. The importance of innate immunity can be appreciated by considering that the generation time of most bacteria is 20 to 30 minutes, whereas the development of a specific adaptive immune response takes days to weeks. The innate immune system protects the host during the time between microbe exposure and initial adaptive responses.

The mechanisms by which the innate and adaptive (acquired) immune systems function are fundamentally different:

The innate immune system recognizes microbes through pattern recognition receptors (PRRs), which are receptors specific for molecular components of microorganisms that are not made by the host. Some innate immune responses are temporarily upregulated as a result of exposure to microbes, but the components of the innate immune system do not change permanently during an individual's lifetime. However, phylogenetic studies have indicated that genes for PRRs and other components have been gradually modified over generations by natural selection [1,5,6].

                                   

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Literature review current through: Nov 2016. | This topic last updated: Tue May 24 00:00:00 GMT 2016.
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