IL-13 (Interleukin-13) plays a key role in the pathogenesis of allergy, cancer, and tissue fibrosis. It is secreted by several helper T cell subsets, NK cells, mast cells, eosinophils, basophils, and visceral smooth muscle cells. IL-13 suppresses the production of proinflammatory cytokines and other cytotoxic substances by macrophages, fibroblasts, and endothelial cells. It promotes B cell activation, immunoglobulin class switching to IgE, and the upregulation of CD23/Fc epsilon RII. IL-13 signals through a receptor complex containing IL-13 R alpha 1 and IL-4 R alpha. This complex also functions as the type 2 IL-4 receptor. IL-13 R alpha 2 binds IL-13 and prevents IL-13 from signaling through the IL-13 R alpha 1/IL-4 R alpha complex. IL-13-bound IL-13 R alpha 2 can directly promote tumor cell invasiveness and the development of tissue fibrosis.