Fig. 1: MALT and mucosal immune response.

The MALT can be functionally divided into 2 portions, the inductive and the effector sites. The organized lymphoid tissue composed of lymphoid follicles, present along the GIT (GALT) and the respiratory tract (NALT) represent the inductive sites where immune response is initiated. Overlying the follicles are specialized epithelium which in the Peyer’s patches is called the follicle-associated epithelium (FAE). This overlying epithelium is equipped with functionally active microfold cells (M cells) which are involved in antigen sampling from the lumen and delivers these luminal antigens to the underlying DCs and macrophages (APCs) in the subepithelial follicles via transcytosis. Some of the underlying DCs and Macrophages also directly sample antigens from the lumen by the extension of transepithelial dendrites across the epithelium or by occasional migration into the lumen. Following antigen capture, the APCs delivers and present the antigens to the T cells and B cells present in the follicles to induce an antigen-specific immune response. The activated T and B cells then exit the submucosa via the lymphatics to the mesenteric lymph nodes where the immune response may be further exaggerated before finally draining into the systemic circulation. These activated cells then express mucosal homing receptors such as CCR9 and CCR10 and are guarded by gradient of chemokines such as CCL25 and CCL28 present in the mucosa to finally exit the blood, a process mediated by integrins and adhesion molecule α4β7 and MAdCAM-1 respectively. At the effector site where the effector functions are carried out, activated T cells go on to become effector cells and/or tissue-resident memory cells. Activated B cells undergo class-switch to become IgA+ B cells and plasma cells which add joining chains to secrete polymeric IgA. These polymeric IgA are transported transcellular to the lumen following binding to polymeric Ig receptor (pIgR) as secretory IgA (sIgA) which lines the mucus and functions in trapping microbes.