Overview of gut immunology.

The gastrointestinal tract (GI tract) plays dual roles in human physiology: digestion and uptake of nutrients and the more daunting task of maintaining immune homeostasis (protecting the body from potentially harmful microbes, while inducing tolerogenic responses to innocuous food, commensals and self-antigens). The unique architecture of the GI tract facilitates both of these functions; multiple levels of infolding results in an immense overall surface area that allows maximal nutrient absorption while housing the largest number of immune cells in the body. This review will focus on how mucosal immune responses generated in the GI tract are organized and controlled. The gastro-intestinal associated lymphoid tissue (GALT), which is composed of discrete inductive and effectors sites, is able to discriminate between harmful and harmless antigens while maintaining homeostasis. Inductive sites are organized into specialized aggregations oflymphoid follicles called Peyer's patches (PP), while effector sites are more diffusely dispersed. The separation of these sites serves to limit and control immune responses. In addition to its distinct architecture, the GI tract has specialized immune cells that aid in promoting a tolerogenic response to orally introduced antigens, (e.g. subsets ofdendritic cells (DCs) and regulatory T-cells (T(R. Secretory IgA (sIgA),which is produced in appreciable quantities at mucosal surfaces, also promotes an anti-inflammatory environment by neutralizing immune stimulatory antigens. The mechanisms of induction tolerance are currently poorly understood; however, this tolerant environment limits potentially damaging inflammatory responses to inappropriate stimuli.