IL-13 is a cytokine that is produced by different T-cell subsets and dendritic cells. IL-13 shares many biologic activities with IL-4. This is due to the fact that IL-13- and IL-4-receptor complexes share the IL-4-receptor alpha-chain, which is important for signal transduction. T cells do not express functional IL-13 receptors. This is the reason why IL-13, in contrast to IL-4, fails to induce TH2-cell differentiation, one of the hallmarks of the allergic response. However, IL-13 is required for optimal induction of IgE synthesis, particularly in situations in which IL-4 production is low or absent. On the other hand, IL-13 inhibits proinflammatory cytokine and chemokine production in vitro and has potent antiinflammatory activities in vivo. From these observations, it can be concluded that IL-13 is an antiinflammatory cytokine that plays a unique role in the induction and maintenance of IgE production and IgE-mediated allergic responses.