Diffuse and Organized Gut Associated Lymphoid Tissue

The gut-associated lymphoid tissue (GALT) represents the largest component of the common mucosal immune system and functions to control intestinal infections. It consists of a diffuse lymphoid compartment containing large populations of lymphocytes and antigen-presenting cells (e.g., macrophages, dendritic cells) in the intestinal lamina propria and Peyer's patches, which are organized lymphoid follicles covered by a single layer of specialized epithelial cells (i.e., M cells). Studies of germ-free and gnotobiotic rodents have shown that microflora play a role in the development of the GALT (Bauer et al. 2006). As the inductive site for mucosal immunity, Peyer's patches play a critical role in sampling of luminal contents and initiating adaptive immune responses towards potentially harmful microorganisms and antigenic materials (Mowat 2003). This includes the generation of immunoglobulin A (IgA)-producing lymphoblasts. These cells mature in the system circulation and then traffic as plasma cells to mucosal effector sites in the gut lamina propria. Neurons and nerve fibers exist in close proximity to lamina propria leukocytes, including mast cells (Wood 2007) and lymphocytes (Downing and Miyan 2000).

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Fig. 5.1 Functional classes of enteric neurons and their major projections to the mucosa, submu-cosal neurons, and smooth muscle coats in the intestinal wall of a large mammal, such as a pig. Note that the mucosa receives both afferent and efferent innervation. CM circular muscle; IPAN intrinsic primary afferent neuron; ISMP and OSMP inner and outer submucosal plexuses; LM longitudinal muscle; MP myenteric plexus; MUC mucosa (from Linden and Farrugia 2008)

Fig. 5.1 Functional classes of enteric neurons and their major projections to the mucosa, submu-cosal neurons, and smooth muscle coats in the intestinal wall of a large mammal, such as a pig. Note that the mucosa receives both afferent and efferent innervation. CM circular muscle; IPAN intrinsic primary afferent neuron; ISMP and OSMP inner and outer submucosal plexuses; LM longitudinal muscle; MP myenteric plexus; MUC mucosa (from Linden and Farrugia 2008)

Peyer's patches are highly innervated (Defaweux et al. 2005; Vulchanova et al. 2007; Chiocchetti et al. 2008). Antigen-specific secretory IgA synthesized in these plasma cells is the major immunoglobulin secreted onto mucosal surfaces and plays an important role in mucosal protection; furthermore, constitutively produced secretory IgA is thought to regulate the gut microfloral population (Suzuki et al. 2007; Macpherson and Slack 2007). In addition to their important immunological role, Peyer's patches are exploited as portals of entry into the body for several species of enteropathogenic bacteria (Clark and Jepson 2003).

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