Innate and acquired plasticity of the intestinal immune system.

The principal function of the intestines is to process food through digestion and to take up and transport the digested nutrients across the epithelium, i.e. from the lumen to the blood stream. The intestinal mucosa constitutes a vast surface area covered by a monolayer of epithelial cells dedicated to the above-mentioned functions. Intestinal epithelia are therefore constantly exposed to large quantities and a wide variety of (i) food antigens and (ii) resident microorganisms forming the commensal flora (about 1011–12 per g of lumenal material in the colon) [1]. From time to time, food and water supplies introduce exogenous pathogenic microbes that are specifically equipped to invade the mucosa [2]. Hence, gut homeostasis requires immunological “ignorance” of the large amount of food antigens and commensal flora as well as effective protection from incessant and abundant environmental challenges and injuries. The aim of the following set of reviews is to describe how innate and adaptive immunity closely discriminates between and regulates hyporesponsiveness or specific immune deviation for “beneficial” antigens (so-called oral tolerance) on one hand and mechanisms which defend against “harmful” antigens on the other.