Enterocytes express proteins associated with immune functions when incubated with proinflammatory cytokines. They express major histocompatibility complex class II molecules associated with antigen presentation and increase the expression of the polymeric immunoglobulin receptor, which is involved in immunoglobulin A secretion. They also acquire the ability to synthesize and secrete acute-phase reactants and cytokines and to express enzymes (cyclooxygenase II and 12-lipoxygenase), involved in the synthesis of lipid mediators of inflammation. Finally, proinflammatory cytokines induce the synthesis of manganese superoxide dismutase, an enzyme that is not associated with an immune function in the enterocyte but rather with enterocyte self-defense against reactive oxygen species, products of immune activation from other cell types. These findings suggest that immune activation affects enterocytes by inducing the expression of genes associated with immune function, allowing the enterocytes to function as accessory immune cells.