Innate And Adaptive Immunity

Defense against microbes is mediated by the early reactions of innate immunity and the later responses of adaptive immunity (Fig. 1-1 and Table 1-2). Innate immunity (also called natural or native immunity) provides the early line of defense against microbes. It consists of cellular and biochemical defense mechanisms that are in place even before infection and are poised to respond rapidly to infections. These mechanisms react to microbes and to the products of injured cells, and they respond in essentially the same way to repeated infections. The principal components of innate immunity are (1) physical and chemical barriers, such as epithelia and antimicrobial chemicals produced at epithelial surfaces; (2) phagocytic cells (neutrophils, macrophages), dendritic cells, and natural killer (NK) cells; (3) blood proteins, including members of the complement system and other mediators of inflammation; and (4) proteins called cyto-kines that regulate and coordinate many of the activities of the cells of innate immunity. The mechanisms of innate immunity are specific for structures that are common to groups of related microbes and may not distinguish fine differences between microbes.

In contrast to innate immunity, there are other immune responses that are stimulated by exposure to infectious agents and increase in magnitude and defensive capabilities with each successive exposure to a particular microbe. Because this form of immunity develops as a response to infection and adapts to the infection, it is called adaptive immunity. The defining characteristics of adaptive immunity are exquisite specificity for distinct molecules and an ability to "remember" and respond more vigorously to repeated exposures to the same microbe. The adaptive immune system is able to recognize and react to a large number of microbial and nonmicrobial substances. In addition, it has an extraordinary capacity to distinguish between different, even closely related, microbes and molecules, and for this reason it is also called specific immunity. It is also sometimes called acquired immunity, to emphasize that potent protective responses are "acquired" by experience. The main components of adaptive immunity are cells called lymphocytes and their secreted products, such as antibodies. Foreign substances that induce specific immune responses or are recognized by lymphocytes or antibodies are called antigens.

Mechanisms for defending the host against microbes are present in some form in all multicellular organisms. These mechanisms constitute innate immunity. The more specialized defense mechanisms that constitute adaptive immunity are found in vertebrates only. Two functionally similar but molecularly distinct adaptive immune systems developed at different times in evolution. About 500 million years ago, jawless fish, such as lampreys and hagfish, developed a unique immune system containing diverse lymphocyte-like cells that may function like lymphocytes in more advanced species and even responded to immunization. The antigen receptors on these cells were variable leucine-rich receptors that were capable of recognizing many antigens but were distinct from the antibodies and T cell receptors that appeared later in evolution. Most of the components of the adaptive immune system, including lymphocytes with highly

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