The mammalian gut microbiota is involved in the intestinal homeostasis and shapes the adaptive immune system. The interaction between TLRs and different ligands (such as polysaccharide A of B. fragilis, serum amyloid A protein of segmented filamentous bacteria, extracellular ATP from intestinal microbiota, flagellin, LPS, and unmethylated CpG of bacterial DNA) induces CD8⁺ T activation and naïve CD4⁺ T polarization towards Th17, Th1, and Treg subsets. The Th17 cells act against pathogens and promote autoimmune disease. The Th1 cells upregulate the production of proinflammatory cytokines, whereas Treg cells produce IL-10 and are involved in the maintenance of homeostasis and in a downregulation of inflammation. Moreover, CD8⁺ T cells induce the IFN-γ and other cytotoxic mediators production.