Epithelial cells and their products are essential participants in both initial and final stages of reactivity to foreign materials in the gut lumen. Follicle-associated epithelial cells, called "M"-cells by others, more efficiently transport antigens from the lumen than do columnar absorptive cells. Epithelial cells degrade antigens intracellularly. The presence of Ia-like antigens on epithelial cells of the gastrointestinal tract and thymus suggests a role in antigen presentation. Actively phagocytic macrophages are present within the epithelium of Peyer's patches and appendix, providing antigen degradation and presentation. Follicle-associated epithelial cells transport materials from lamina propria into intestinal lumen. Therefore, FAE cells are capable of bidirectional transport. Plasma cells are located within and immediately beneath the epithelium of Peyer's patches and appendix, where defects in the basal lamina indicate diminished capability for molecular selection. Transmission of antibodies into the lumen would modulate further antigen uptake and provide for interaction with organisms.