Immunologists have shown that the GALT not only re-sponds to antigenic stimuli but also functions in a monitor-ing capacity. This function has been partially clarified for the lymphatic nodules of the intestinal tract. The M cells that cover Peyer’s patches and lymphatic nodules have a distinctive surface that might be misinterpreted in sections as thick microvilli. The cells are readily identified with the scanning electron microscope because microfolds con-trast sharply with the microvilli that constitute the striated border of the adjacent enterocytes. It has been shown with horseradish peroxidase (an enzyme used as an experimental marker) that the M cells pinocytose protein, from the intestinal lumen, transport the pinocytotic vesicles through the cell and discharge the protein by exocytosis into deep recesses of the adjacent extracellular space (Fig. F17.5.1). Lymphocytes within the deeply recessed extracellular space sample the luminal protein, including antigens, and thus have the opportunity to stimulate development of specific antibodies against the antigens. The destination of these exposed lympho-cytes has not yet been fully determined. Some remain within the local lymphatic tissue, but others may be des-tined for other sites in the body, such as the salivary and mammary glands. Recall that in the salivary glands, cells of the immune system (plasma cells) secrete IgA, which the glandular epithelium then converts into sIgA. Some exper-imental observations suggest that antigen contact neces-sary for the production of IgA by plasma cells occurs in the lymphatic nodules of the intestines. bb M cells lymphocytes absorptive cells lamina propria a FIGURE F17.5.1 • Diagram of M cells in a lymphatic nodule of the intestine. a. This diagram shows the relationship of the M cells (microfold cells) and absorptive cells to the lymphatic nodule. The M cell is an epithelial cell that displays microfolds rather than microvilli on its apical surface. It has deep recesses within which lymphocytes come close to the lumen of the small intestine. M cells have MHC II molecules on their surface and are therefore considered antigen-presenting cells. Antigen from the intestinal lumen is presented to T lymphocytes residing within the recesses of the M cell. (Based on Owen RL, Nemanic PC, eds. Scanning Electron Microscopy. Vol II. O’Hare, IL: AMF, 1978.) b. Scanning electron micrograph of a Peyer’s patch lymphatic nodule bulging into the lumen of the ileum. Note that the area of the follicle covered by M cells is surrounded by the fingerlike projections of the intestinal villi. The surface of the M cells has a smooth appearance. The absence of absorptive cells and mucus-producing goblet cells in the area covered by M cells facilitates immunoreactions to antigens. 80. (Reprinted with permission from Owen RL, Johns AL. Epithelial cell specialization within human Peyer’s patches: an ultrastructural study of intestinal lymphoid follicles. Gastroenterology 1974;66:189–203.) tissue samples the antigens in the epithelial intercellular spaces. Lymphocytes and other antigen-presenting cells process the antigens and migrate to lymphatic nodules in the lamina propria where they undergo activation (see page 444), leading to antibody secretion by newly differentiated plasma cells.
Mucosal surface is protected by immunoglobulinmediated responses.