English: The gastrointestinal tract defenses. (A) The gastrointestinal tract is composed of numerous cell types that are important for immune activation and barrier surface defenses. The gastrointestinal epithelium is composed of enterocytes, goblet cells, Paneth cells, enteroendocrine cells, tuft cells, and stem cells. In contrast, the lamina propria is composed of immune cells such as dendric cells, T cells, and macrophages. (B) Polarized intestinal epithelial cells have distinct apical and basolateral domains. The apical domain contains microvilli and is closely associated with the actin cytoskeletal network.