Cultivation of epithelial-associated microbiota by the immune system.

Mounting evidence supports the intuitive idea that many of the factors produced in defense of the epithelial surface, including mucin and secretory IgA, promote the growth of the commensal microbial flora, much the same as plant-derived mucoid substances support the growth of symbiotic microbes in the rhizosphere associated with roots. Thus, the 'defense' of the host epithelial surface often involves support and maintenance of microbial growth, despite an unfortunate tendency to view the immune system as an antagonist to the microbial flora. The perspective that the immune system supports the growth of a symbiotic microbiota has the potential to push forward our understanding of host-microbe interactions and to facilitate the development of new treatments for diseases associated with the microbiota.