Immune System

Abstract

Abstract The immune system comprises an interacting assemblage of cells and soluble molecules whose primary function is to kill invading microorganisms that may cause damage to the body. Two interdependent kinds of immune system are present in most vertebrates, which together trigger one or more different killing mechanisms according to whether the microbes live within or outside the cells of the body. An innate system, mediated by receptors which recognize uniquely microbial structures, responds rapidly to the threat of invading organisms. This underlies an adaptive system, mediated by antigen receptors on lymphocytes, which produces a more sustained and comprehensive response. Only the adaptive system, found exclusively in vertebrates, retains a memory of exposure to each microbe and ensures the system is mobilized more rapidly upon a subsequent infection by the same pathogen. Key concepts A rapidly mobilizable form of defence (the immune system) is required to kill microbes that breach the skin and other passive defences of the body. Physical injury to tissues triggers inflammation which serves to mobilize the immune system and to repair tissue damage. The innate immune system is moblized first in response to microbes and is triggered by receptors for microbial components that are not found in the body. The innate system comprises cells such as phagocytes (macrophages, neutrophils) and natural killer cells, and soluble antimicrobial components such as complement. The adaptive immune system is mediated by T and B lymphocytes, which are activated through the recognition of microbial components using antigen receptors. The genes for antigen receptors are produced by a unique process of gene rearrangement that results in a vast repertoire of antigen receptors, each with a unique antigen specificity. The repertoire of antigen receptors present in the body ensures that any invading microbe missed by the innate system can be recognized by the adaptive system. The adaptive system uses many of the same killing (effector) mechanisms as the innate system, and is likely to have evolved only in vertebrates. Antibodies are soluble forms of the B lymphocyte antigen receptor that are produced in large quantities during an adaptive immune response; one functionof antibodies is to bind to microbes and render them more susceptible to phagocytosis. Because each lymphocyte has a different receptor, the adaptive system is mobilized more gradually than the innate system; however, the response is more sustained, less easily evaded by microbes, and unlike the innate system, it retains a memory of exposure to a particular microbe to ensure a more rapid response to the same microbe upon subsequent exposure.