Interleukin 33 (IL-33) is a cytokine belonging to the IL-1 family. Similarly to other IL-1 family members, IL-33 exists in both an un-cleaved (full-length) and a cleaved form, although the proteins responsible of IL-33 maturation are still under investigation. IL-33 is expressed by several cell types including epithelial cells, endothelial cells, and fibroblasts, and binds a specific receptor, also known as IL1RL1, which is expressed on Tʜ2 cells and mast cells. Signaling via IL1RL1 involves receptor accessory protein (IL1RAP) and leads to the expression of type 2 cytokines, such as IL-4, IL-5, IL-13. Via enhancing the activation of Tʜ2 cells, mast cells, and eosinophils, IL-33 sustains the humoral response and contributes to adverse Tʜ2 reactions, such as dermatitis, asthma, and allergies. Besides its primary involvement in the immune system modulation, IL-33 is also expressed in blood epithelium and smooth muscle and plays a role in bone protection. It is unclear whether also the immature form of IL-33 could be biologically active after release, such as during necrosis. Moreover, it was suggested that the full length protein could co-localize in the nucleus and act as transcriptional factor. Recombinant Human IL-33 corresponds to the mature form of human IL-33.