Anatomy of the Lymphoid System

The lymphoid system is a collection of tissues and organs that are strategically designed and located to bring the population of B cells and T cells into contact with any and all antigens that enter the body (figure 16.2). This is important because lymphocytes are highly specific, recognizing only one or a few different antigens. In order for the body to mount an effective response, the appropriate lymphocyte must actually encounter the given antigen. ■ tissues, p. 70 ■ organs, p. 70

Lymphatic Vessels

Flow within the lymphoid system occurs via the lymphatic vessels, or lymphatics. These vessels carry a fluid called lymph, which is collected from the fluid that bathes the body's tissues. This fluid is formed as a result of the body's circulatory system (see figure 28.1). As oxygenated blood travels from the heart and lungs through the capillaries, much of the fluid portion is extruded into the surrounding tissues, supplying them with the oxygen and nutrients that are carried by the blood. The majority of the fluid then reenters the capillaries as they return to the heart and lungs, but some enters the lymphatic vessels instead. The lymph, which may contain antigens that have entered the tissues, travels via the lymphatics to the lymph nodes, where materials including protein and cells are removed. The lymph then empties back into the blood circulatory system at a large vein behind the left collarbone. Note that the inflammatory response results in greater accumulation of fluid in the tissues at the site of inflamma

Figure 16.2 Anatomy of the Lymphoid System Lymph is distributed through a system of lymphatic vessels, passing through many lymph nodes and lymphoid tissues. For example, lymph enters a lymph node (inset) through the afferent lymph vessels, percolates through and around the follicles in the node, and leaves through the efferent lymphatic vessels.The lymphoid follicles are the site of cellular interactions and extensive immunologic activity.

SALT (skin associated lymphoid tissue)

Bone marrow tion; this causes a corresponding increase in the antigen-containing fluids that enter lymphatic vessels.

Secondary Lymphoid Organs

Secondary lymphoid organs are the sites where lymphocytes gather to contact the various antigens that have entered the body. Examples include the lymph nodes, spleen, the tonsils, adenoids, and appendix. They are situated at strategic positions in the body so that defensive immune responses can be initiated at almost any location. For example, lymph nodes capture materials from the lymphatics, and the spleen seizes materials from blood.

The secondary lymphoid organs are like busy, highly organized lymphoid coffee shops where many cellular meetings take place and information is exchanged. The anatomy of the organs provides a structured center to facilitate the interactions and transfer of cytokines between various cells of the immune system, including lymphocytes, dendritic cells, and macrophages. No other places in the body have the structure necessary to facilitate the complex interactions required, which is why these organs are the only sites where productive adaptive immune responses can be initiated.

Tonsils

Thymus

Axillary lymph node

MALT (mucosal associated lymphoid tissue) Breast, oral, respiratory, gastrointestinal, and genitourinary tract tissues

Tonsils

Thymus

Axillary lymph node

MALT (mucosal associated lymphoid tissue) Breast, oral, respiratory, gastrointestinal, and genitourinary tract tissues

Bone marrow

Lymph node

When lymphocytes make contact with a given antigen and receive the required "second opinion", they respond by proliferating to form clones of cells specific for that antigen. The metabolically active and dividing lymphocytes have larger nuclei and more abundant cytoplasm than their resting counterparts. The familiar lymph node ("gland") swelling that occurs in nodes draining a site of infection, as in the neck area during an episode of sore throat, represents the dividing lymphoid cells actively engaged in an adaptive immune response against the offending agent causing the infection.

Some secondary lymphoid organs are less organized in structure than the lymph nodes and spleen, but their purpose remains the same—to capture antigens, bringing them into contact with lymphocytes that have gathered. Among the most important of these lymphoid organs are the Peyer's patches, which sample antigens collected from the small intestine, allowing presentation of the antigen to lymphocytes below the mucosal surface (see figure 19.6). M cells, specialized epithelial cells lying over the Peyer's patches, collect material in the intestine and transfer it to the lymphoid tissues beneath. Peyer's patches are part of a network of lymphoid tissues called mucos-al-associated lymphoid tissue (MALT). These play a critical role in mucosal immunity, the element of adaptive immunity that prevents microbes from entering the body via the mucous membranes. Collections of lymphocytes under the skin are called skin-associated lymphoid tissue (SALT).

Primary Lymphoid Organs

The bone marrow and thymus are the primary lymphoid organs. This is where the hematopoietic stem cells destined to become B cells and T cells mature. Both B cells and T cells originate in the bone marrow but only B cells mature there; immature T cells migrate to the thymus. Once mature, the lymphocytes gather in the secondary lymphoid organs just described, waiting to encounter antigen. ■ hematopoietic stem cells, p. 376

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