Interleukin-33 (IL-33) is a dual-function cytokine belonging to the IL-1 family. It is constitutively expressed and stored in the nuclei of barrier tissue cells (e.g., epithelial and endothelial cells). Upon cellular damage or stress (e.g., from allergens, pollutants, or infection), IL-33 is rapidly released as an "alarmin". It then acts on immune cells expressing its receptor ST2 (mainly group 2 innate lymphoid cells (ILC2s), Th2 cells, and mast cells), triggering a potent cascade of type 2 inflammation characterized by the production of cytokines like IL-4, IL-5, and IL-13. This makes IL-33 a central upstream regulator in diseases such as asthma, COPD, and atopic dermatitis.