[Gut microbiota and development of the immune system].

During their long co-evolution, bacteria and their animal host have developed mutualistic interactions that are regulated by the immune system of the host. A dialogue between bacteria and the host immune system is initiated at birth during microbial colonization. This colonization induces the recruitment of multiple immune cell types that cooperate with the intestinal epithelium to construct a barrier capable of confining the microbes within the intestinal lumen. Regulatory mechanisms avoid deleterious inflammatory reactions that would harm both the host and its microbiota. In mouse, homeostatic activation of the intestinal immune system is recapitulated by a small number of bacteria, and more particularly by the segmented filamentous bacteria.