Whether we are aware or not, diverse microorganisms are living on almost all environmentally exposed surfaces on our body without eliciting harmful immune responses. In fact, recent understanding from numerous studies indicates that our health is highly dependent on the contribution of intestinal commensal bacteria. It appears through its symbiotic interaction with the host, which is the result of millions of years of co-evolution, the microbiota shapes the immune system. In this review, we discuss the relationship between host physiology and commensal bacteria and explore the molecular mechanisms by which the adaptive immune system is influenced by the intestinal microbiota.