Interactions between commensal intestinal bacteria and the immune system

Although we might shudder at the thought of billions of bacteria living in our lower intestine, we are colonized by these passengers shortly after birth. However, the relationship is mostly of mutual benefit, and they shape our immune system throughout life. Here, we describe our developing understanding of the far-reaching effects that the commensal flora have on mucosal and systemic immunity and their relevance to the effects of hygiene on human disease.