Human Interleukin-33 (IL-33) is a cytokine that plays an important role in regulating immune responses and inflammatory signaling in humans (Homo sapiens). IL-33 is a member of the IL-1 cytokine family and is produced primarily by epithelial cells, endothelial cells, and fibroblasts, particularly in barrier tissues such as the skin, lungs, and gastrointestinal tract. It functions as an “alarmin,” being released upon cellular damage or stress to alert and activate the immune system. Human IL-33 binds to the ST2 receptor (IL1RL1) together with the IL-1 receptor accessory protein (IL-1RAcP), activating intracellular signaling pathways such as NF-κB and MAPK that promote production of inflammatory mediators. IL-33 plays a key role in promoting type 2 immune responses by activating T helper 2 (Th2) cells, group 2 innate lymphoid cells (ILC2s), mast cells, eosinophils, and basophils, leading to the production of cytokines such as IL-4, IL-5, and IL-13. Through these actions, IL-33 contributes to regulation of allergic inflammation, tissue repair, and host defense against parasites. In humans, IL-33 has been strongly associated with inflammatory and allergic diseases including asthma, atopic dermatitis, allergic rhinitis, and other Th2-mediated disorders, and it is widely studied in immunology and translational biomedical research as a therapeutic target for inflammatory and immune-mediated diseases.