The Immune System

General Features

Our ability to coexist with the vast array of microbes present in our environment is a measure of the efficacy of various defense systems to monitor insults to the internal cellular environment from the extracellular world. Arguably the most important of these defense systems is the immune system. In essence the immune system is the instrument by which the body discriminates self from nonself, and destroys nonself. Important functional elements of the immune system include several cell types, tissues, serum proteins, and small peptides such as chemokines and cytokines. While components of the immune system interact with one another, detailed discussion is simplified by classification into two categories, innate immunity and adaptive immunity.

The innate immune system is made up of cells and molecules that function early in the protective response to a foreign substance (antigen), using primitive receptors found also within the invertebrate world to distinguish self from nonself.

I There is no unique antigenic specificity that triggers the elements of the innate immune system, which, we shall see later, makes this system quite different from the "more developed" adaptive immune system (dependent upon lymphocytes). Furthermore, these innate responses do not confer any host advantage upon subsequent exposure to the same microbe. In other words, innate immunity does not lead to immunological memory; again this is in sharp contrast to lymphocyte-mediated immune function.

From a phylogenetic point of view, the adaptive immune response is a more recent development than innate immunity. However, various components of each system have been co-opted into use by the other system. Thus, recruitment of cells during adaptive immune responses, and their activation, is often contingent upon the action of cytokines generated during innate immunity. Adaptive immune responses, in turn, lead to the production, by specific cells, of effector molecules including cytokines, and other serum proteins called antibodies. Antibodies and cytokines, themselves, often contribute to the effector function of cells functioning within innate and/or adaptive immune processes. In contrast to the quick response time for innate immune processes, naive cells that mediate adaptive immunity require several days to become formidable effectors. Lymphocytes are restricted in their recognition repertoire, with each cell responding only to one unique portion (epitope) of an antigen.

Hence, while the immune system is conveniently considered to provide two modes of host defense, in reality innate and adaptive immune responses act in concert, but differ with respect to: (i) specificity; (ii) immunological memory, i.e., the ability to "recall" previous exposure to antigen; and (iii) constancy of response time and magnitude of the response (Table 1.1).

Innate Immune Responses

Our first lines of defense against infectious agents are the natural physical and chemical barriers that prevent microbes from invading our bodies. Intact skin and mucosal linings are effective physical barriers unless they are compromised. Chemical barriers also serve as deterrents to infectious agents. As example, lysozyme present in secretions splits the cell walls of gram positive bacteria; spermine (in semen) prevents the growth of gram positive bacteria; the acid pH of the

Table 1.1. Characteristics of the innate and the adaptive immune system

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