IL-33 protein is a key cytokine belonging to the IL-1 family, functioning as an alarmin that plays crucial roles in tissue homeostasis, pathogenic infection, inflammation, allergy, and type 2 immunity. It is abundantly expressed in endothelial cells, epithelial cells, and fibroblast-like cells. IL-33 transmits signals through its receptor ST2, which is expressed on the surface of T helper 2 (Th2) cells and group 2 innate lymphoid cells (ILC2s), inducing transcription of Th2-associated cytokine genes and host defense against pathogens. In innate immunity, IL-33 activates ILC2s, basophils, eosinophils, and mast cells to drive early immune responses against allergens and other environmental insults. In adaptive immunity, it regulates the functions of dendritic cells (DCs) and influences the development of Th2, Tfh, and other immune cells. IL-33 is released upon cellular damage or stress and can enhance the differentiation of recently activated T cells. It is also involved in the development of multiple types of immune-related diseases, making it a promising therapeutic target for modulating immune responses in conditions such as asthma, allergic diseases, and autoimmune disorders.