Porth's Pathophysiology, 9e

Chapter 13 Innate and Adaptive Immunity    301

in the cortex under the influence of thymic hormones and ­cytokines. As the T cells mature, they develop the TCRs that differentiate them from other types of T cells. The majority of the thymocytes die in the cortex during the process of gene rearrangement and maturation because they fail to develop the appropriate receptor types on their cell membranes. Only those T cells capable of recognizing foreign antigen displayed by self-MHC are allowed to mature. This process is called thymic selection . Mature, immunocompetent T-helper and T-cytotoxic cells leave the thymus in 2 to 3 days and enter the peripheral lymphoid tissues through the bloodstream. Lymph Nodes Lymph nodes are small aggregates of lymphoid tissue located along lymphatic vessels throughout the body. The lymphatic vessels carry lymph, which is a clear sometimes yellow- tinged fluid that contains a variety of white blood cells (pre- dominantly lymphocytes) and transports cellular debris and organisms to the lymph modes to be removed from the body. Each lymph node processes lymph from a discrete, adjacent anatomic site. Lymph nodes are congregated in the axillae and groin and along the great vessels of the neck, thorax, and abdomen. The lymph nodes receive lymph from the col- lecting ducts, which ultimately drain into the thoracic duct located in the left side of the chest at the level of the sub- clavian vein. Lymph nodes have two functions: removal of

foreign material from lymph before it enters the bloodstream and serving as centers for proliferation and response of immune cells. Lymph nodes are bean-shaped, encapsulated tissues, approximately 0.5 to 1 cm in diameter. Lymph enters the node through afferent lymph channels and leaves through the effer- ent lymph vessels located in the deep indentation of the hilus. Lymphocytes and macrophages move slowly through the lymph nodes so that they have adequate time to engulf micro- organisms and interact with circulating antigen. The lymphatic system provides a large surface upon which macrophages and DCs can more easily present antigens to T lymphocytes. Lymph nodes are divided into three distinct and spe- cialized areas—an outer cortex, a paracortex, and an inner medulla (Fig. 13.14). The T lymphocytes predominate in the paracortex and the B lymphocytes predominate in the follicles and germinal centers of the outer cortex. The T lymphocytes proliferate when antigens enter the paracortex of the lymph node. They then migrate to the outer cortex so that they can interact with B lymphocytes that are stored there. Within the follicles the lymphocytes continue to mature, replicate, and interact with the PACs present in the nodes (macrophages and follicular DCs). Activated B cells then migrate to the medulla of the lymph node, where they complete their maturation into plasma cells. Large quantities of antibodies are then released into the systemic circulation.

Cortex

Medulla Paracortex Follicles

Afferent vessel

Trabecula

Reticular fibers

Germinal center

Lymphoid sinuses

Capsule

Nodule

Hilus

Medullary cord

FIGURE 13.14  •  Structural features of a lymph node. Bacteria that gain entry to the body are filtered out of the lymph as it flows through the node.

Efferent vessel