Lymphocytes and other immune-system cells such as macrophages and dendritic cells are found throughout the intestinal tract, both in organized tissues and scattered throughout the surface epithelium of the mucosa and in the underlying layer of connective tissue called the lamina propria. The organized secondary lymphoid tissues in the gut comprise a group of organs known as the gut-associated lymphoid tissues (GALT), together with the draining mesenteric and caudal lymph nodes (Fig. 12.5). The GALT and the mesenteric lymph nodes have the anatomically compartmentalized structure typical of peripheral lymphoid organs, and are sites at which immune responses are initiated. The cells scattered throughout the epithelium and the lamina propria comprise the effector cells of the local immune response.
The intestinal mucosa of the small intestine to mesenteric lymph node isolated lymphoid follicle Intestinal lumen Peyer’s patch subepithelial dome crypt follicle (B cells) T-cell area lamina propria lamina propria lymphocyte epithelium intraepithelial lymphocyte lymphatic villus Scattered lymphoid cells Organized lymphoid tissues Intestinal lymphocytes are found in organized tissues where immune responses are induced, and scattered throughout the intestine, where they carry out effector functions
Fig. 12.5 Gut-associated lymphoid tissues and lymphocyte populations.
is made up of fingerlike processes (villi) covered by a thin layer of epithelial cells (red) that are responsible for digestion of food and absorption of nutrients. These epithelial cells are replaced continually by new cells that derive from stem cells in the crypts. The tissue layer underlying the epithelium is called the lamina propria, and is colored pale yellow throughout this chapter. Lymphocytes are found in several discrete compartments in the intestine, with the organized lymphoid tissues such as Peyer’s patches and isolated lymphoid follicles forming what is known as the gut‑associated lymphoid tissues (GALT). These tissues lie in the wall of the intestine itself, separated from the contents of the intestinal lumen by the single layer of epithelium. The draining lymph nodes for the gut are the mesenteric lymph nodes, which are connected to Peyer’s patches and the intestinal mucosa by afferent lymphatic vessels and are the largest lymph nodes in the body. Together, these organized tissues are the sites of antigen presentation to T cells and B cells and are responsible for the induction phase of immune responses. Peyer’s patches and mesenteric lymph nodes contain discrete T‑cell areas (blue) and B‑cell follicles (yellow), while the isolated follicles comprise mainly B cells. Many lymphocytes are found scattered throughout the mucosa outside the organized lymphoid tissues: these are effector cells—effector T cells and antibody‑secreting plasma cells, as well as innate lymphoid cells (ILCs). Effector lymphocytes are found both in the epithelium and in the lamina propria. Lymphatics also drain from the lamina propria to the mesenteric lymph nodes.