Immunologic structures and functions of the gut.

The intestine is richly populated with lymphoid tissue capable of initiating and effecting a wide variety of immunologic reactions. These reactions have consequences not only for the gut itself but for the body in general, and have established the importance of the gut as an immunologic organ. Among the outer and inner surfaces of our body, the 200 to 300 m2 of the gut contrast with the 2 m2 of the skin, and the 80 m2 of the lung. At the inner surface of the intestine, our organism contacts intimately bacteria, parasites, enzymes, toxins, a wide variety of dietary substances and their breakdown products. The essential barrier against the permanent antigenic burden is the mucosa. Its integrity depends on the continual replication, maturation, and metabolism of its constituents. Additional defense functions are exerted by the mucus, lysozyme, phagocytes, other cells, humoral factors and biological response modifiers involved in inflammatory and immune reactions. Some of these factors are being produced very close to the surface at which they act. The sum of the mechanical, humoral, cellular, immunologic and non-immunologic defense factors of the intestinal mucosa constitutes the mucosal block. However, the block is not complete. Rather, a continuous antigenic uptake through the epithelial layer takes place. The specialized structures of Peyer's patches, solitary lymph follicles, appendix vermiformis and their associated epithelium allow a controlled antigen uptake (sampling). Because of the heavy antigenic load, the intestine can be described as the most important immunologic contact organ of our body. The antigens may give rise to local and systemic immune reactions with antibody production, or the suppression of systemic immunologic responses to ingested antigens ("oral tolerance").