Gut microbiota and systemic immunity in health and disease.

The mammalian intestine is colonized by trillions of microorganisms that have co-evolved with the host in a symbiotic relationship. Although the influence of the gut microbiota on intestinal physiology and immunity is well known, mounting evidence suggests a key role for intestinal symbionts in controlling immune cell responses and development outside the gut. Although the underlying mechanisms by which the gut symbionts influence systemic immune responses remain poorly understood, there is evidence for both direct and indirect effects. In addition, the gut microbiota can contribute to immune responses associated with diseases outside the intestine. Understanding the complex interactions between the gut microbiota and the host is thus of fundamental importance to understand both immunity and human health.