Interleukin-33 Receptor subunit, ST2 (IL1RL1). ST2 exists in two main isoforms: a transmembrane form (ST2L) that forms part of the functional IL-33 receptor complex (with IL-1RAcP), and a soluble decoy receptor (sST2). IL-33 is an alarmin cytokine primarily released from epithelial and endothelial cells upon cellular damage or stress. It acts as a potent initiator and amplifier of type 2 immune responses. Binding of IL-33 to ST2L on immune cells (such as group 2 innate lymphoid cells (ILC2s), Th2 cells, mast cells, and regulatory T cells) triggers MyD88-dependent signaling, leading to the production of type 2 cytokines (IL-4, IL-5, IL-13) and other inflammatory mediators. In chronic airway diseases like COPD and asthma, the IL-33/ST2 axis is central to sustaining inflammation, mucus production, and tissue remodeling.