IL-13 is an immunoregulatory cytokine produced mainly by T helper 2 cells, but also by mast cells and NK cells. IL-13 is a unique cytokine having similar structural features and functional characteristics as IL-4, but only shares ~30% homolog with the IL-4 sequence. IL-13 inhibits IL-1β, IL-6, IL-8, and TNF-α resulting in anti-inflammatory effects on monocytes and macrophages. IL-13 also induces B cells to proliferate and modulating TH2 cell development. Recombinant rat IL-13 is an 11.9 kDa protein containing 109 amino acid residues.