IL-33/ST2 signaling pathway. IL-33 is the ligand for ST2. It activates the ST2L/IL-1RAcP dimers or is neutralized by binding to sST2. The interaction of IL-33 with ST2 leads to the recruitment of the myeloid differentiation primary-response protein 88 (MyD88), IL-1R-associated kinase 1 (IRAK1) and IRAK4, which result in the activation of at least two independent pathways: the transcription factor nuclear factor-κB (NF-κB) and the mitogen-activated protein kinase (MAPK) pathway, ultimately induce related gene expression. It induces the production of IL-2, IL-5, IL-13, TNF-α, and IFN-γ. IL-33 can also combine with single Ig IL-1R-related molecule (SIGIRR), which seems as an inhibitor of IL-33/ST2 pathway.