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INTRODUCTION

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Our immune system has evolved to provide protection from the pathogens that inhabit the natural world, and a critical aspect of immunologic function is the distinction between self tissues and non-self tissues. Self tissues must not become a target for immune responses, and training to protect self tissues is the essence of tolerance. Our host defense includes barrier functions, and we now know that the barrier interacts dynamically with the cells of the immune system as part of this defense. In this overview, we will describe the elements of the immune system, the development of different functional arms of the immune system, and the beautiful choreography that is intrinsic to an immune response.

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Lymphocytes, neutrophils, monocytes, and various other myeloid cells can be detected in the blood through simple microscopy. Immunologists identify subsets of these cells through the use of flow cytometry. The cells usually are categorized by the expression of certain cell surface molecules, which are referred to as a cluster of differentiation (CD) molecules. A convenient table for reference relevant to clinical immunology and immunologic disorders is given in Table 182-1.

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Table Graphic Jump Location
TABLE 182-1CLUSTER OF DIFFERENTIATION (CD) CLASSIFICATION AND FUNCTION OF SELECTED MOLECULES
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ONTOGENY OF ...

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