FIGURE 14.10 • Macrophage activation by a helper CD4 T cell. Helper CD4 T lymphocytes recognize the bacterial antigen expressed in the context of MHC II molecules on the surface of a macrophage that has phagocytosed the bacteria. The recognition of MHC II molecules activates the T cell, which in turn secretes IL-2. IL-2 acts as an autocrine hormone to stimulate T-cell division and differentiation. Newly formed helper CD4 T lymphocytes also interact with MHC II molecules and release interferon  (IFN-). This cytokine stimulates the macrophage to destroy the bacteria inside its phagosomes. CD4 molecules on the surface of the T cell also potentiate antibacterial reactions.
tissue that are not enclosed by a capsule. Lymphocytes and other free cells of this tissue are found in the lamina propria (subepithelial tissue) of these tracts. This form of lymphatic tissue is called diffuse lymphatic tissue or mucosa-associated lymphatic tissue (MALT) because of its association with mucous membranes (Fig. 14.12). These cells are strategically located to intercept antigens and initiate an immune response. After contact with antigen, they travel to regional lymph nodes, where they undergo proliferation and differentiation. Progeny of these cells then return to the lamina propria as effector B and T lymphocytes.
The importance of diffuse lymphatic tissue in protecting the body from antigens is indicated by two factors:  The regular presence of large numbers of plasma cells, especially in the lamina propria of the gastrointestinal tract, a morphologic indication of local antibody secretion.
 The presence of large numbers of eosinophils, also frequently observed in the lamina propria of the intestinal and respiratory tracts, an indication of chronic inflammation and hypersensitivity reactions.
Lymphatic nodules are discrete concentrations of lymphocytes contained in a meshwork of reticular cells.
In addition to diffuse lymphatic tissue, localized concentrations of lymphocytes are commonly found in the walls of the alimentary canal, respiratory passages, and genitourinary tract. These concentrations, called lymphatic nodules or lymphatic follicles, are sharply defined but not encapsulated (Fig. 14.13). A lymphatic nodule consisting chiefly of small lymphocytes is called a primary nodule. However, most nodules are secondary nodules and have distinctive features that include the following:  A germinal center is located in the central region of the nodule (Fig. 14.14) and appears lightly stained in histologic sections. The germinal center develops when a lymphocyte that has recognized an antigen returns to a primary nodule and undergoes proliferation. The lighter
FIGURE 14.12 • Photomicrograph of diffuse lymphatic tissue.
This photomicrograph shows the diffuse lymphatic tissue in the lamina propria (LP) of the large intestine. The lower portion of two intestinal glands (Gl) is also evident. The highly cellular, diffuse lymphatic tissue includes fibroblasts, plasma cells, and eosinophils. However, the most abundant cell component, whose presence characterizes diffuse lymphatic tissue, is the lymphocyte, which can be identified by its small, round, dark-staining nucleus.
320.