The anatomical basis for the immune function of the gut.

The barrier function of the gut wall can be divided into different histotopographically defined lines of defence. These consist not only of lymphoid cells but also of goblet cells, entero-endocrine cells, macrophages and mast cells. Subsets of lymphoid cells are found preferentially within the epithelium (T suppressor) or in the lamina propria (T helper). Most plasma cells produce IgA. Peyer's patches are described in detail as typical organized lymphoid structures of the gut. In man, they are present well before birth and are found in large numbers even in old age. They are not only typical for the ileum but are also present in the duodenum and jejunum. The four compartments in Peyer's patches, i.e. follicle, corona, interfollicular area and the dome, are defined by the typical localization of lymphocyte subsets and by their different functions. Typical features of the epithelium of the dome are the lack of villi and goblet cells and the presence of specialized epithelial cells (M cells) which are important for the uptake of particulate antigen from the gut lumen. Precursor cells of IgA producing plasma cells leave the intestinal wall via the lymphatics and return preferentially to the gut mucosa, and this is summarized by the term gut-associated lymphoid tissue (GALT). Other organs with mucous membranes, such as mammary and salivary glands, bronchial and genital tract, are also included in this circulatory route and this is expressed by the term mucosa-associated lymphoid tissue (MALT). Mast cells in the gut mucosa can be classified as connective tissue or mucosa mast cells. These differ in their sensitivity to formaldehyde as a fixative, contain different granules and mediators, have different origins, and show major differences in the effectiveness of antiallergic compounds on the stabilizing of the cell membrane. Mucosa mast cells have also been demonstrated in the human gut. The histotopographical relationship of many cell types such as goblet and M cells in addition to cells of the immune system such as lymphoid cells, macrophages and mast cells, is essential in the understanding of the barrier function of the gut wall.