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Cells Present in the Intestine

As already mentioned, the GIT is the largest interface between the body and the environment, and, for this reason, it also serves as a point of communication between the environment and the host immune system (Brandtzaeg 2011; Faria et al. 2013). This interface consists of a single epithelial layer folded into crypts and villi to increase the surface area of the gut (Cummins and Thompson 2002; Ismail and Hooper 2005). The colon does not contain villi. The intestinal epithelial layer is composed of several distinct cell types, originating from multipotent stem cells present in the crypts (see Fig. 2). The most abundant are the enterocytes that have an absorptive function. Interlaced between the enterocytes are mucin-secreting goblet cells and peptide hormone exporting enteroendocrine cells (see also Part IV) (Ismail and Hooper 2005; Snoeck et al. 2005). During their migration to the top of the villi, enterocytes, goblet cells and enteroendocrine cells differentiate and eventually die (apoptosis) when they reach the top of the villi. A fourth cell type, the Paneth cells, migrates downwards to the crypt base. The Paneth cells secrete digestive enzymes, growth factors and antimicrobial peptides (AMPs) such as cryptdins or defensins (Snoeck et al. 2005). For more information on epithelial cells and ex vivo cell systems, see Part II and Part IV.

Given its large surface area and the number of antigens and microorganisms to which the GIT is exposed, it is not surprising that it contains the highest number of lymphoid (immune) cells in the entire body (Faria et al. 2013). The gut immune system consists of inductive sites where antigen recognition and primary adaptive immune responses take place and effector sites that harbour, amongst other cells, activated Tand B-cells and memory cells. The main inductive sites are gutassociated lymphoid tissues (GALT) such as Peyer's patches (PP), isolated lymphoid follicles (ILF) and the mesenteric lymph nodes (mLNs). The lamina propria (LP) and the epithelium constitute the main effector sites (Pabst and Mowat 2012). In the GALT and at the effector sites, a wide range of immune cells are present (Faria et al. 2013). Dendritic cells (DCs), which are recognised as an important link between innate and adaptive immunity, take up, process and present antigens to T-cells (Willart et al. 2013). DCs are found in all organised intestinal lymphoid tissues. In the sub-epithelial dome (SED) of Peyer's patches, they capture antigens that are transported into the SED by specialised epithelial cells called microfold cells (M-cells) (Shreedhar et al. 2003). Next to DCs a mixture of T and B lymphocytes, plasma cells and macrophages, a second type of antigen-presenting cells, are present in the SED (Pabst 1987). In the lamina propria (LP), a range of different immune cells can be found, typically DCs, macrophages, plasma cells, memory Band T-cells, mast cells, eosinophils and cytotoxic natural killer cells (NK cells) (Macdonald and Monteleone 2005; Peterson and Artis 2014). Also, LP contains additional innate immune cell populations not found in peripheral blood. These cells, called innate lymphoid cells (ILC), are potent cytokine producers, much like the classical T helper cell subsets. Recent evidence suggests important functions of ILCs in the maintenance of barrier integrity and mucosal homeostasis. The only immune cells that are virtually absent in the healthy intestine are neutrophils and basophils. Both cell types, however, will infiltrate intestinal tissues in case of inflammation (Stone et al. 2010; Ismail and Hooper 2005). Generally, the small intestine contains more immune cells than the colon. More details on the different immune cell types present in the intestine can be found in Part III.

Fig. 2 Intestinal cells and structures in the small intestine. Note: Normally you will not find the mesenteric lymph node in the submucosa, but in humans they are situated close to the serosa on the mesenteric side of the gut. But for simplicity and to be complete, we added the node in the picture

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