Mouse IL-33 Carrier-Free Recombinant Protein, eBioscience?. IL-33 (Interleukin-33) is a 270 amino acid, highly divergent protein belonging to the IL-1 family with an IL-1-like C-terminal domain. IL-33 is a dual function protein that may function both as a proinflammatory cytokine and an intracellular nuclear factor with transcriptional regulatory properties. IL-33 binds to and signals through IL1RL1/ST2 and its stimulation recruits MYD88, IRAK1, IRAK4, and TRAF6. IL-33 activates NF-kappaB and MAP kinases, and drives production of TH2-associated cytokines from in vitro polarized TH2 cells. In vivo, IL-33 induces the expression of IL-4, IL-5, and IL-13 and leads to severe pathological changes in mucosal organs. IL-33 is proteolytically