Layers of mutualism with commensal bacteria protect us from intestinal inflammation

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Abstract
The immune system of the intestine is phylogenetically ancient, and its development appears to have preceded most other lymphoid structures, including the thymus.1 The intestinal mucosal immune system is also of immense size. Approximately 70% of all lymphocytes in the body are within the mucosal immune system and most of the antibody produced in healthy individuals is IgA, which is secreted across mucous membranes. Intestinal immunity must be finally balanced between the capacity for mounting protective immune responses to infectious agents and yet tolerate the enormous load of antigens and immunostimulatory molecules that constitute the commensal intestinal bacteria.