This review is a brief survey of our knowledge of the immune functions of the gastrointestinal tract. The mucosal immunologic system is present at all epithelial surfaces that are in direct contact with the external environment, is largely independent of the systemic immune response, and is governed by antigenic stimuli at epithelial surfaces. The plasma cell population responsive to these antigens is found just below the epithelium, and the antibodies produced are transported to the epithelial surface by a unique process. Finally, the antibodies secreted are able to function even in the presence of proteolytic enzymes. Brief reviews of immunophysiology and immunomorphology of the gastrointestinal immune system are enclosed. The immune response of the gut is discussed in relation to immune regulation by helper and suppressor systems. Even though secretory immunity may be considered "old hat," several new aspects of this system are emphasized along with an overview of the system. The clinical manifestations and pathophysiology of the two major adult immunodeficiency syndromes are reviewed in order to emphasize normal immunophysiology. There is a brief discussion of food allergy. Future directions that may be appropriate for research in both systemic and mucosal immunology are discussed in the final section.