The immune system of the gastrointestinal tract (gut-associated lymphoid tissue--GALT) differs from the peripheral immune system in a number of points, and can also react largely independently of the latter. The lymphatic cells of GALT are both strictly compartmentalized (Peyer's patches, lymphatic follicles), and diffusely distributed within the mucosa. The organized lymphatic tissue represents the afferent component, the diffuse lymphatic tissue the efferent component, of the intestinal immune response. A marked recirculation behavior (homing) of the intestinal lymphocytes makes it clear hat GALT is simply part of a more comprehensive common mucosal immune system, the mucosa-associated lymphatic tissue (MALT). At the center of a description of the pathophysiology of the small bowel as an immune organ is Crohn's disease. By way of example, current immunological aspects of immunoregulation, specific and unspecific cytotoxic, that is, tissue-injurious, immune reactions, as well as immunoglobulin isotype and immunoglobulin G subclass differentiation are discussed.