Fig. 1.24 Organization of a Peyer’s patch in the gut mucosa. As the diagram on the left shows, a Peyer’s patch contains numerous B-cell follicles with germinal centers. the areas between follicles are occupied by t cells and are therefore called the t-cell dependent areas. the layer between the surface epithelium and the follicles is known as the subepithelial dome, and is rich in dendritic cells, t cells, and B cells. Peyer’s patches have no afferent lymphatics, and the antigen enters directly from the gut across a specialized epithelium made up of so-called microfold (m) cells. Although this tissue looks very different from other lymphoid organs, the basic divisions are maintained. As in the lymph nodes, lymphocytes enter Peyer’s patches from the blood across the walls of high endothelial venules (not shown), and leave via the efferent lymphatic. the light micrograph in panel a shows a section through a Peyer’s patch in the gut wall of a mouse. the Peyer’s patch can be seen lying beneath the epithelial tissues. gC, germinal center; tdA, t-cell dependent area. Panel b, a scanning electron micrograph of the follicle-associated epithelium boxed in panel a, shows the m cells, which lack the microvilli and the mucus layer present on normal epithelial cells. each m cell appears as a sunken area on the epithelial surface. Panel c, a higher-magnification view of the boxed area in panel b, shows the characteristic ruffled surface of an m cell. m cells are the portal of entry for many pathogens and other particles. Panel a, hematoxylin and eosin stain, magnification ×100; panel b, ×5000; panel c, ×23,000.
1-17 Mucosal surfaces have specialized immune structures that orchestrate responses to environmental microbial encounters.
Most pathogens enter the body through mucosal surfaces, and these are also exposed to a vast load of other potential antigens from the air, food, and the natural microbial flora of the body. Mucosal surfaces are protected by an extensive system of lymphoid tissues known generally as the mucosal immune system or mucosa-associated lymphoid tissues (MALT). Collectively, the mucosal immune system is estimated to contain as many lymphocytes as all the rest of the body, and they form a specialized set of cells obeying somewhat different rules of recirculation from those in the other peripheral lymphoid organs. The gut-associated lymphoid tissues (GALT) include the tonsils, adenoids, appendix, and specialized structures in the small intestine called Peyer’s patches, and they collect antigen from the epithelial surfaces of the gastrointestinal tract. In Peyer’s patches, which are the most important and highly organized of these tissues, the antigen is collected by specialized epithelial cells called microfold or M cells (Fig. 1.24). The lymphocytes form a follicle consisting of a large central dome of B lymphocytes surrounded by smaller numbers of T lymphocytes. Dendritic cells resident within the Peyer’s patch present the antigen to T lymphocytes. Lymphocytes enter Peyer’s patches from the blood and leave through efferent lymphatics. Effector lymphocytes generated in Peyer’s patches travel through the lymphatic system and into the bloodstream, from where they are disseminated back into mucosal tissues to carry out their effector actions.