Figure 3 | Mucosal Immunology

Figure 3

From: Terminology: nomenclature of mucosa-associated lymphoid tissue

Figure 3

Delineation of lymphoid-cell distribution in various compartments of the gut wall with some species differences indicated. Lymphocytes can leave the gut wall via draining lymphatics afferent to mesenteric lymph nodes (MLNs), or via portal blood reaching the liver where important regulation of immunity apparently takes place, particularly induction of tolerance. Commonly used abbreviations (see Table 2) are shown for various aggregates of lymphoid cells such as Peyer's patches (PPs). The frequency of M cells (not indicated see Fig. 1) in the follicle-associated epithelium of PPs is highly variable among different species, and a reliable marker for these specialized epithelial cells has not been identified in humans. Note also that, in contrast to the antigen-dependent priming of B cells that takes place in PPs of mammals and birds, the continuous ileal PP present in ruminants, pigs, and dogs appears to be a primary lymphoid organ responsible for antigen-independent B-cell development, similar to the bursa of Fabricius in chicken (not indicated). This PP can be up to 2 m long and constitute 80–90% of the intestinal lymphoid tissue. Reptils and fish do not have PPs. In these species, the gut immune system is composed of leukocytes scattered in the epithelium and occasional small non-organized lymphoid aggregates. (Modified from Brandtzaeg and Pabst1 with permission from Elsevier.)

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