The tissues of the immune system consist of the generative (also called primary, or central) lymphoid organs, in which T lymphocytes and B lymphocytes mature and become competent to respond to antigens, and the peripheral (or secondary) lymphoid organs, in which adaptive immune responses to microbes are initiated. The principal generative lymphoid organs are the thymus, where T cells develop, and the bone marrow, the site of production of all blood cells and where B lymphocytes mature. The major peripheral organs are briefly described in the following sections.
The peripheral lymphoid organs are organized to concentrate antigens, APCs, and lymphocytes in a way that optimizes interactions among these cells and the development of adaptive immune responses. Most of the body’s lymphocytes are located in these organs (
Table 5.1
Lymph nodes are encapsulated, highly organized collections of lymphoid cells and innate immune cells that are located along lymphatic channels throughout the body (
Fig. 5.8A ). As lymph passes through lymph nodes, resident APCs are able to sample antigens that are carried to the node in lymph derived from the interstitial fluids of tissues. In addition, DCs transport antigens from nearby epithelial surfaces and tissues by migrating through lymphatic vessels to the lymph nodes. Thus, antigens (e.g., of microbes that enter through epithelia or colonize tissues) become concentrated in draining lymph nodes.
The spleen has an important role in immune responses to bloodborne antigens. Blood entering the spleen flows through a network of sinusoids, which enables the
Fig. 5.8 Morphology of a lymph node. (A) The histology of a lymph node, with an outer cortex containing follicles and an inner medulla. (B) The segregation of B cells and T cells in different regions of the lymph node, illustrated schematically. (C) The location of B cells (stained green, using the immunofluorescence technique) and T cells (stained red) in a lymph node. (Courtesy of Drs. Kathryn Pape and Jennifer Walter, University of Minnesota School of Medicine, Minneapolis, Minnesota.) trapping of bloodborne antigens by resident DCs and macrophages.
• The cutaneous and mucosal lymphoid systems are located under the epithelia of the skin and the gastrointestinal and respiratory tracts, respectively. They respond to antigens that enter through breaches in the epithelium. Pharyngeal tonsils and Peyer’s patches of the intestine are two anatomically defined mucosal lymphoid tissues. The large number of lymphocytes in mucosal organs (second only to lymph nodes) reflects the huge surface area of these organs.
Within the peripheral lymphoid organs, T lymphocytes and B lymphocytes are segregated into different regions (
Fig. 5.8B, C ). In lymph nodes, the B cells are concentrated in discrete structures, called follicles, located around the periphery, or cortex, of each node. If the B cells in a follicle have recently responded to an antigen, the follicle develops a central pale-staining region called a germinal center. The T lymphocytes are concentrated in the parafollicular cortex. The follicles contain the FDCs that are involved in the activation of B cells, and the paracortex contains the DCs that present antigens to T lymphocytes. In the spleen, T lymphocytes are concentrated in periarteriolar lymphoid sheaths surrounding small arterioles, and B cells reside in the follicles.