Evidence now exists for bidirectional communication between the three key factors in the GI tract: Diet, Immune and commensal microflora. Diet can have profound influence on the immune system (e.g. Vitamin A, Vitamin D, AHR), while the immune system can also affect dietary intake. Diet also has dominant influence on the composition and metabolic capacity of commensal bacteria, while this, in return, influences nutrient absorption. The immune system is able to exert control over both commensal composition and localization, while commensal signals are critical for development and function of the immune system.