IMMUNOLOGY - Roitt I. - Mir 2000

Chapter 3. The Lymphoid System

■ Lymphoid Organs and Tissues are classified as either primary (central) or secondary (peripheral). The Primary lymphoid organs are the Thymus and the Cytology/practical/86.html">Red Bone Marrow.

■ Lymphocytes differentiate from lymphoid stem Cells in the primary lymphoid organs and migrate to Secondary Lymphoid Organs and tissues to perform their Functions.

■ Secondary lymphoid organs include the Spleen and Lymph Nodes. In addition, regions of secondary lymphoid tissue (lymphoid aggregates) are present within mucous membranes, forming a unified system known as the mucosa-associated lymphoid tissue (MALT).

■ The Immune Response to Antigens entering the body through mucous membranes is initiated by the priming of lymphocytes, primarily within Peyer's patches.

■ Different lymphoid organs protect different body systems: the spleen responds to antigens circulating in the Blood; lymph nodes react to antigens arriving via Lymphatic vessels; and MALT protects the mucous membranes.

■ Most lymphocytes are not sedentary, but circulating cells; they constantly migrate from the bloodstream into lymphoid tissues and re-enter the blood via the Thoracic duct.

To function with maximum efficiency, the cells involved in the immune response operate as part of specialized tissues and organs that together make up the lymphoid system.

LYMPHOID TISSUES ARE COMPONENTS OF PRIMARY AND SECONDARY LYMPHOID ORGANS AND STRUCTURES

The functional Cells of the lymphoid system consist of lymphocytes, accessory cells (macrophages and antigen-presenting cells), and, in certain tissues, epithelial cells. All of these cells function either within discrete, capsule-enclosed lymphoid organs or as diffuse structures. The major lymphoid organs and tissues are subdivided into primary (central) and secondary (peripheral) ones (Fig. 3.1).

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Fig. 3.1. The primary lymphoid organs are the thymus and bone marrow, where T AND B cells mature, respectively. Cellular and humoral immune responses arise within secondary (peripheral) lymphoid organs and structures. Secondary lymphoid organs can be classified According to the body regions (systems) whose immune defense they provide. The spleen responds predominantly to antigens circulating in the bloodstream. Lymph nodes develop immune reactions to antigens delivered via lymph, i.e., those that have penetrated through the Skin (supervising peripheral lymph nodes) or through mucous membranes (internal organ lymph nodes). Tonsils, Peyer's patches, and other mucosal lymphoid structures (blue rectangles) react to antigens that have breached surface barriers. The red bone marrow functions as both a primary and a secondary lymphoid organ.

Primary lymphoid organs. Primary lymphoid organs serve as the primary site of lymphocyte development. Here, lymphocytes differentiate from lymphoid stem cells, proliferate, and mature into functional cells. In mammals, T cells mature in the thymus, while B lymphocytes mature in the fetal Liver and bone marrow (see Ch. 12). Birds have a specialized site of B-Cell development, the bursa of Fabricius. It is within the primary lymphoid organs that the repertoire of specificities for lymphocyte antigen-receptors is generated, thereby equipping lymphocytes to recognize any antigen the body may encounter during its lifetime. Subsequently, these cells undergo Selection for self-tolerance to autoantigens, after which, in peripheral lymphoid organs or structures, they recognize foreign antigens exclusively. In the thymus, T cells also "learn" to recognize self-MHC molecules (see Ch. 12). At the same time, it is known that certain types of lymphocytes develop outside the primary lymphoid organs.

Secondary lymphoid organs. Secondary lymphoid organs and structures include the spleen, lymph nodes, and mucosa-associated lymphoid tissue, including the tonsils of the pharyngeal ring and Peyer's patches of the ileum. Secondary lymphoid tissue provides the microenvironment in which lymphocytes can interact with antigens, with one another, and with accessory cells. The ensuing immune response requires phagocytic macrophages, antigen-presenting cells, and mature T and B lymphocytes.