FIGURE 14.13 • Photomicrograph of a lymphatic nodule. This photomicrograph shows a section of the wall of the small intestine (duodenum). Short villi and intestinal glands are present in the upper part of the micrograph. A lymphatic nodule (LN) occupies most of the remainder of the micrograph. The lighter central region of the nodule is the germinal center. The lymphocytes in the germinal center are larger than those in the denser region of the nodule. They have more cytoplasm, so, their nuclei are more dispersed, giving the appearance of a less compact cellular mass. 120.
staining is attributable to the large immature lymphocytes (lymphoblasts and plasmablasts) that it contains. These lymphocytes have large amounts of dispersed euchromatin in their nuclei rather than the dense heterochromatin of small lymphocytes. Follicular dendritic cells (FDCs) are also present in germinal centers interdispersed between populations of B lymphocytes. The germinal center is a morphologic indication of lymphatic tissue response to antigen. The presence of a germinal center represents a cascade of events that includes activation and proliferation of lymphocytes, differentiation of plasma cells, and antibody production. Mitotic figures are frequently observed in the germinal center, reflecting the proliferation of new lymphocytes at this site. The number of FDCs and macrophages in the germinal center often increases dramatically after a period of intense response to an antigen.
 A mantle zone or corona is present that represents an outer ring of small lymphocytes that encircles the germinal center.
Lymphatic nodules are usually found in structures associated with the alimentary canal such as the tonsils, ileum, and vermiform appendix.
Generally, nodules are dispersed singly in a random manner. In the alimentary canal, however, some aggregations of nodules are found in specific locations. These include the following:  Tonsils form a ring of lymphatic tissue at the entrance of the oropharynx. The pharyngeal tonsils (adenoids) (located in the roof of the pharynx), the palatine tonsils (or simply the tonsils, located on either side of the pharynx and between the palatopharyngeal and palatoglossal arches), and the lingual tonsils at the base of the tongue all contain aggregates of lymphatic nodules. The palatine tonsils consist of dense accumulations of lymphatic tissue located in the mucous membrane. The squamous epithelium that forms the surface of the tonsil dips into the underlying connective tissue in numerous places, forming tonsillar crypts (Fig. 14.15). The walls of these crypts usually possess numerous lymphatic nodules. Like other aggregations of lymph nodules, tonsils do not possess afferent lymphatic vessels; however, lymph drains from the lymphatic tissue of the tonsil via efferent lymphatic vessels.
 Peyer’s patches are located in the ileum (distal portion of the small intestine) and consist of numerous aggregations of lymphatic nodules containing T and B lymphocytes (Fig. 14.16). In addition, numerous isolated single (solitary) lymph nodules are located along both large and small intestines.
 The vermiform appendix arises from the cecum. The lamina propria is heavily infiltrated with lymphocytes and contains numerous lymphatic nodules. Although the appendix is often described as a vestigial organ, the abundant lymphatic tissue that it contains during early life suggests that it is functionally associated with bursa-equivalent organs.