[The bowel: a key component of the immune system].

The gut is constantly exposed to a high antigenic load coming from the diet and commensal bacteria. The Gut-Associated Lymphoid Tissue (GALT) constitutes the most extensive and complex part of the immune system and is capable of efficiently distinguishing invasive pathogens from innocuous antigens. The knowledge of its unique structure consisting on organised tissue, inductor of the immune response (Peyer's patches and mesenteric lymph nodes), and diffused tissue, effector of the immune response (intraepithelial lymphocytes and lamina propria lymphocytes), allow us to understand the development and regulation of the immune response in the gut and how this one can be extended to the rest of the organism.