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Host–microbe interactions: the difficult yet peaceful coexistence of the microbiota and the intestinal mucosa

The immune system has evolved to live in a collaborative relationship with the microbiota, while still serving its seminal function to fight off invasive pathogenic bacteria. The mechanisms that rule the interactions between the intestinal microbiota and the intestinal immune system are the focus of intense research. Here, we describe how the innate immunity is, to a great extent, in charge of the control of the microbiota in the intestine and relies on non-specific receptors called pathogen-recognition receptors. While the microbiota has a well-defined effect on the host immune homoeostasis, it has become clear that the opposite is also true, i.e., the mucosal immune system has the capacity to shape the microbial population. The mechanisms that rule the reciprocal regulation between host immunity and commensal bacteria (including specific bacteria) are currently being elucidated and will be described here. A better knowledge of how the host and bacteria interact and how the intestinal microbiota and the immune system are co-regulated will provide the basis for a better understanding of intestinal and systemic immunopathologies and for the development of new therapeutic approaches.