Schematic representation of gut-associated lymphoid tissue (GALT), with organized lymphoid structures — Peyer's patches and isolated lymphoid follicles (ILFs) — and diffuse tissue of the epithelium and the lamina propria. Peyer's patches and ILFs are composed of a specialized follicle-associated epithelium (FAE) containing M cells, a subepithelial dome (SED) rich in dendritic cells (DCs), and B-cell follicle(s) that contain germinal centres (GCs), where follicular B cells efficiently undergo class-switch recombination (CSR) and somatic hypermutation (SHM). Migration of B cells into the mucosa takes place through high endothelial venules (HEVs), located in the interfollicular regions of Peyer's patches, which contain mostly T cells. The diffuse tissues of the lamina propria contain a large number of immunoglobulin A (IgA)+ plasma cells, T and B cells, macrophages, dendritic cells (DCs) and stromal cells (SCs). Lamina-propria DCs take up antigens from the lumen and present them directly to T cells and B cells, which can induce IgA class-switching and differentiation in situ. Secreted IgA is transported across the epithelium, where it serves as a first line of defence against pathogens and for the maintenance of gut-flora homeostasis. IgA+ B cells and plasma cells are shown in red, IgG+ cells in blue and IgM+ cells in pink.