The gastrointestinal immune system (gut-associated lymphoid tissue) has the unique capacity to discriminate between harmless and potentially dangerous material. It can raise a protective response against pathogenic microbes and toxins while tolerating food antigens and commensal microbes.

This is a challenge given the vast number of foreign antigens, mainly derived from food (>100 g of protein per day), and commensal microbes colonizing the gut (an estimated 100 trillion, 10 times the number of cells in the human body).

Dysfunction of this delicate balance between immunity and tolerance can lead to pathologies such as food allergy, autoimmune diseases, and infections. Researchers compared normal mice, mice lacking microbes, and mice lacking microbes that were fed an elemental diet devoid of dietary antigens.

They show that dietary antigens trigger the generation of a regulatory T (T_reg) cell type in the small intestine that suppresses immune responses to food. These T_reg cells are phenotypically and functionally distinct from those in the colon that suppress immune responses to commensal microbes.

These intestinal T_reg cells have a limited life span, and repress underlying strong immunity to ingested protein antigens.

http://science.sciencemag.org/content/351/6275/858