Equally, evolution of the mammalian immune system has coincided with the acquisition of a complex microbiota, demonstrating a symbiotic relationship between the host immune system and its commensal microbiota. A self-reinforcing, dynamic dialogue ensures that commensal colonization occurs as a state of mutualism, the breakdown of which can result in chronic inflammatory disorders, including autoimmunity, allergies and metabolic syndromes. Conversely, selective modulation of the microbiota presents immense therapeutic potential for bolstering tumor immunotherapy, vaccination and resistance to antibiotic-resistant microbes.