Resident commensals shaping immunity.

All animals coexist with myriad commensal microorganisms in a symbiotic relationship that plays a key role in health and disease. Continuous commensal-host interactions profoundly affect the development and regulation of the host's immune system. The complex interaction of the commensal microbiota with the immune system is a topic of substantial interest. An understanding of these interactions and the mechanisms through which commensal microbes actively shape host immunity may yield new insights into the pathogenesis of many immune-mediated diseases and lead to new prophylactic and therapeutic interventions. This review examines recent advances in this field and their potential implications not just for the colonized tissues but also for the entire immune system.