Interleukin 13 (IL-13) is a T cell derived cytokine involved in the regulation of inflammatory and im-mune responses. IL-13Rα1 together with IL-4Rα forms a functional receptor for both IL-4 and IL-13, which is why these two cytokines share many of their biological activities. The receptor is found on human B cells, monocytes and endothelial cells. However, no functional receptor is expressed on T cells, which explains why IL-13, in contrast to IL-4, fails to induce TH2-cell differentiation.