Cells and Tissues of the Immune System

• Macrophages are phagocytes that are constitutively present in tissues and respond rapidly to microbes that enter these tissues.

• Neutrophils, an abundant type of phagocyte, and monocytes, the precursors of tissue macrophages, are always present in the blood and can be quickly delivered anywhere in the body.

• Specialized tissues, called peripheral lymphoid organs, function to concentrate microbial antigens that are introduced through the common portals of entry (skin and gastrointestinal and respiratory tracts). The capture of antigen and its transport to lymphoid organs are the first steps in adaptive immune responses. Antigens that are transported to lymphoid organs are displayed by antigen-presenting cells (APCs) for recognition by specific lymphocytes.

• Almost all tissues contain dendritic cells, which are APCs that are specialized to capture microbial antigens, to transport them to lymphoid tissues, and to present them for recognition by lymphocytes.

• Naive lymphocytes (lymphocytes that have not previously encountered antigens) migrate through these peripheral lymphoid organs, where they recognize antigens and initiate adaptive immune responses. The anatomy of lymphoid organs promotes cell-cell interactions that are required for antigen recognition by lymphocytes and for the activation of naive lymphocytes, resulting in the generation of effector and memory lymphocytes.

• Effector and memory lymphocytes circulate in the blood, home to peripheral sites of antigen entry, and are efficiently retained at these sites. This ensures that immunity is systemic (i.e., that protective mechanisms can act anywhere in the body).

Immune responses develop through a series of steps, in each of which the special properties of immune cells and tissues play critical roles. This chapter describes the cells, tissues, and organs that compose the immune system. In Chapter 3, we describe the traffic patterns of lymphocytes throughout the body and the mechanisms of migration of lymphocytes and other leukocytes.

TABLE 2-1 Normal Blood Cell Counts

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