Depiction of the human mucosal immune system. Inductive sites for mucosal immunity are constituted by regional MALT with their B-cell follicles and M-cell (M)-containing follicle-associated epithelium through which exogenous antigens are transported actively to reach APCs, including DCs, macrophages, B cells, and FDCs. In addition, quiescent intra- or subepithelial DCs may capture antigens at the effector site (exemplified by nasal mucosa in the middle) and migrate via draining lymphatics to local/regional lymph nodes where they become active APCs, which stimulate T cells for productive or downregulatory (suppressive) immune responses. Naive B and T cells enter MALT (and lymph nodes) via HEVs. After being primed to become memory/effector B and T cells, they migrate from MALT and lymph nodes to peripheral blood for subsequent extravasation at mucosal effector sites (exemplified by gut mucosa on the right). This process is directed by the local profile of vascular adhesion molecules and chemokines, the endothelial cells thus exerting a local gatekeeper function for mucosal immunity. The gut lamina propria contains few B lymphocytes but many J-chain-expressing IgA (dimers/polymers) and IgM (pentamers) plasmablasts and plasma cells. Also, there are normally some rare IgG plasma cells with a variable J-chain level (J), and many T cells (mainly CD4+). Additional features are the generation of SIgA and SIgM via pIgR (mSC)-mediated epithelial transport, as well as paracellular leakage of smaller amounts (broken arrow) of both locally produced and plasma-derived IgG antibodies into the lumen. There may also be some active transport of IgG mediated by the neonatal Fc receptor (not indicated). Note that IgG cannot interact with J chain to form a binding site for pIgR. The distribution of intraepithelial lymphocytes (mainly T-cell receptor α/β+CD8+ and some γ/δ+ T cells) is also depicted. The inset (lower left corner) shows details of an M cell and its “pocket” containing various cell types. The cartoon is modified from Brandtzaeg and Pabst1 with permission from Elsevier. APCs, antigen-presenting cells; DCs, dendritic cells; FDCs, follicular dendritic cells; HEVs, high endothelial venules; MALT, mucosa-associated lymphoid tissue; mSC, membrane secretory component; pIgR, polymeric Ig receptor; SIgA, secretory IgA; SIgM, secretory IgM.