Today, there is a growing interest in the intestinal microbiota and its relationship with the host's immunity. This is mainly due to two causes: first, the results obtained with probiotics, which have been defined as live micro-organisms that confer a health benefit on the host when consumed in adequate amounts (1), have shown interesting immunomodulatory properties in humans (1-3). Second, the studies by Dutchmann and coworkers (4) demonstrated for the first time, some years ago, that we are tolerant to our own digestive flora. A breakdown of this state leads to inflammatory bowel diseases (IBD). Consequently, the digestive flora can be considered as an organ belonging to the host's just as the spleen, heart, or brain. It plays an important role in the host's protection, especially by its actions on the immune system.

The overall importance of the intestine, relating to health, is still not completely understood. It is an extremely complex organ, which has to assure the function of digestion of foods and absorption of nutrients. In addition to this, the intestine is the largest lymphoid organ in the body by virtue of lymphocyte number and quantity of immunoglobulin produced. It also harbors a huge reservoir of bacteria that colonize it very early after birth and which is called "the commensal or resident or autochthonous digestive microflora," and more recently the "intestinal microbiota." The relationships between the intestinal microbiota and intestinal immune system (IIS), described in some reviews (5-7) can be viewed in terms of "symbiosis" or "mutualism," which is the association of symbiosis and commensalism as explained by Hooper and Gordon (8). Indeed the IIS does not mount immune responses toward the intestinal microbiota that, in turn, exert many effects on the immune system. These effects can be characterized as activation, modulation, and regulation of immune responses and are effective at both intestinal and peripheral levels.

In this chapter, effects of the intestinal microbiota on the host's immunity will be described, and in some cases effects of probiotic bacteria will also be discussed.

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