Anatomy and Function of the Gut Immune System

The intestinal immune system is faced with a difficult task of appropriately responding to the complex variety of foreign substances in the digestive tract, including innocuous dietary antigens, the commensal microbiota, and potential pathogens. To manage this task the intestinal immune system uses a network of organized and diffuse lymphoid tissues that coordinate responses to maintain homeostasis. Aggregates of immune cell populations of various sizes and compositions are scattered along the entire length of the small and large intestines, and function as induction sites for immune responses directed toward the luminal content. In addition to these organized lymphoid structures, the tissue compartments in closest proximity to the luminal content, the epithelium, and the underlying lamina propria contain a large number of effector cells that act locally to maintain and promote intestinal barrier function as well as sampling luminal antigens and migrate to the mesenteric lymph nodes to induce antigen-specific responses to acquired antigens. In this article we will review the anatomical organization and current understanding of how the different structures and compartments contribute to maintaining intestinal homeostasis.