Figure 1.

Anatomical distribution of intestinal T cells. T cells distributed within the small and large intestine are frequently arranged within the gut-associated lymphoid tissue (GALT), which are composed of organized lymphoid tissues including mesenteric lymph nodes, Peyer's patches, and isolated lymphoid follicles (ILFs) as well as more diffusely scattered lymphocytes including the lamina propria lymphocytes and intraepithelial lymphocytes (IELs). IELs reside within the epithelium layer and are proximity to antigens in the gut lumen, making them components of the front line of immune defense against invading pathogens. There are more IELs in the small intestine compared with the colon. The majority of T cells in lamina propria are CD4⁺ T cells, with only a small population of CD8⁺ T cells. Among the CD4⁺ T cell subsets, Th17 cells and Treg cells are the most abundant cells in the gut, conferring protection against fungi and maintenance of intestinal homeostasis. The organization of Peyer's patches is comparable to that of lymph nodes, with large B cell follicles and T cell areas. Peyer's patches are in close contact with microfold cells (M cells) located in gut epithelium which can capture and transport antigens from the lumen to antigen-presenting cells such as dendritic cells (DCs) in the underlying Peyer's patches. DCs can also form transepithelial dendrites to directly sample luminal antigens and then emigrate to the mesenteric lymph nodes, where they activate naive T cells to induce gut-tropic T cells. The ILFs, with features similar to Peyer's patches, are distributed along the whole intestinal tract and believed to provide a complementary system for Peyer's patches.