Clinical Concept

Immune System

Characteristic

Innate Immune System

Adaptive Immune System

Immunological Memory

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Specificity

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Response Time Constancy

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stomach prevents colonization of most bacteria. Once infectious agents have penetrated these physical and chemical barriers, nonspecific mechanisms of host defense are induced in an attempt to eliminate the intruders. Phagocytes, natural killer cells, inflammatory cells, and antigen presenting cells are the principal cells that function in innate immunity (Fig. 1.1).

In addition to the induction of cellular aspects of innate immunity, a system of serum proteins, the complement system, is intimately involved in this form of immunity. Following the deposition of a spontaneously generated fragment of a serum complement protein onto a microbial membrane, particularly bacterial cell membranes, the complement system is activated. Triggering complement activation signals a cascade of activation of complement component proteins, leading to the generation of various protein fragments with distinct biological activities that contribute to host defense. In addition, cytokines released during induction of cellular immunity, in conjunction with antigen presentation by antigen presenting cells, recruit the adaptive immune system, which enhances host defense. The innate immune system, therefore, is characterized by responses that lack memory and specificity, but display constancy with respect to magnitude and rate of the response to antigen.

Adaptive Immune Responses

Adaptive immune responses (also known as acquired immune responses) are mediated by lymphocytes (Fig. 1.2). The system is "adaptive" in that, even before exposure to the pathogen, lymphocytes have been developed that recognize an infinite array of antigens. While there are millions of each lymphocyte type (T and B) present, each antigen is recognized by only a few (in the limit one) cells. In fact, each lymphocyte responds to only one unique portion (epitope) of an antigen. Effective immunity following exposure to a pathogen requires multiple copies of the lymphocytes recognizing the pathogen. As a result, specific lymphocytes undergo binary division (proliferation) that leads to the generation of numerous progeny specific for an epitope on the pathogen. Because the progeny respond to the same antigenic epitope as that of the parent, the parent and the progeny constitute what is termed a clone (Fig. 1.3). Clonal expansion is crucial to generate effective immunity against pathogens during first exposure (primary response), thus ensuring that a sufficient number of lymphocytes specific for that pathogen are available. After the pathogen is cleared not all of the cells that encountered antigen die. Rather the immune system retains a supply of memory cells, which can respond much more quickly to rechallenge (secondary response) with the same antigen that induced the clonal expansion. This constitutes the process of antigen-specific immunological memory.

Adaptive immune responses, in turn, lead to the production of effector molecules including cytokines, and other serum proteins, called antibodies or immu-noglobulins, by specific cells. These effector molecules themselves often contribute to innate and/or adaptive immune processes. As an example, particular antibodies complexed with antigen can trigger the activation of the complement system.

Fig. 1.1. Concept map depicting cells of the innate immune system. The main cell types in innate immunity are phagocytes, natural killer cells, inflammatory cells, and antigen presenting cells. Each has a particular role in innate immunity.

Cells of the Innate Immune System

Fig. 1.1. Concept map depicting cells of the innate immune system. The main cell types in innate immunity are phagocytes, natural killer cells, inflammatory cells, and antigen presenting cells. Each has a particular role in innate immunity.

Antigen Presenting Cells Concept Map

Engulf and Destroy Antigens (e.g. microbes) CD4+ T lymphocytes

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