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Lymph nodes are small ovoid structures (normally less than 1 cm in diameter) found in various regions throughout the body (see Figure 2.2). The human body contains hundreds of lymph nodes located deep inside the body. They are close to major junctions of the lymphatic channels, which are connected to the thoracic duct. The thoracic duct transports lymph and lymphocytes to the vena cava, the vessel that carries blood to the right side of the heart (see Figure 2.14), from where it is redistributed throughout the body.

Lymph nodes are composed of a medulla, with many sinuses, and a cortex, which is surrounded by a capsule of connective tissue (Figure 2.5A). The cortical region contains primary lymphoid follicles. After antigenic stimulation, these structures enlarge to form secondary lymphoid follicles with germinal centers containing dense populations of lymphocytes (mostly B cells) that are undergoing mitosis (Figure 2.5B). In response to antigen stimulation, antigen‐specific B cells proliferating within these germinal centers also undergo a process known as affinity maturation to generate clones of cells with higher affinity receptors (antibody) for the antigenic epitope that triggered the initial response (see Chapter 9). The remaining antigen‐nonspecific B cells are pushed to the outside to form the mantle zone. The deep cortical area or paracortical region contains T cells and dendritic cells. Antigens are brought into these areas by dendritic cells, which present antigen fragments (peptides) to T cells, events that result in activation of the T cells. The medullary area of the lymph node contains antibody‐secreting plasma cells that have traveled from the cortex to the medulla via lymphatic vessels.