[The mucosa-associated immune system--immune defense in the gastrointestinal tract]

The intestinal immune system has to protect the mucosa against microbial and toxic agents. This is achieved by elimination of potential antigens, which are inhibited to adhere and to invade the mucosa. Thus, induction of inflammatory processes is prevented. Elimination of antigens is mediated by the secretion and transport of IgA (and especially IgA2) to the lumen of the gut. There, IgA is the most efficient part of the so-called "mucosa block". The composition of mononuclear cells in the lamina propria exhibits the preference of a memory T cell type that is able to provide help for B cells. If IgA-mediated immune response proves not to be sufficient, IgG-mediated inflammatory processes are induced within the mucosa. The role of cytotoxic T cells and, moreover, the biological function of intraepithelial lymphocytes are still controversial.