IL-13 is a 132-amino-acid nonglycosylated cytokine that shares common features with IL-4. It is secreted by several helper T cell subsets, NK cells, mast cells, eosinophils. 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 bound IL-13 R alpha 2 with high affinity and can directly promote tumor cell invasiveness and the development of tissue fibrosis. Nevertheless, like IL-4, IL-13 inhibits macrophage production of TNF, IL-1β, and pro-inflammatory chemokines, but can upregulate the synthesis of IL-12 by DCs and macrophages.