A simplified diagram showing the possible mechanisms of intestinal bacteria in influencing the polarization of T_H17 cells in the lamina propria. Activation of dendritic cells by intestinal microbes results in secretion of proinflammatory cytokines such as IL-12, L-23, IL-27. T_H17-inducing bacteria may promote T_H17 immunity via IL-23 induction, which may involve signaling mediated by the TLR ligands (a), extracellular ATP (b), and SAA (c). Meanwhile, some probiotic strains may skew immunity away from T_H17 via IL-12 and IL-27 induction as a result of activating TLR and dectin receptors (a′). These cytokines can inhibit T_H17 development while facilitate T_H1 differentiation. Probiotic may also work by controlling the growth and colonization of T_H17-inducing bacteria (d). IL: interleukin; P2X: ionotropic receptors; P2Y: metabotropic receptors; PAMPs: pathogen-associated molecular patterns; SAA: serum amyloid A; TLR: toll-like receptor.