ST2 is a member of the Toll/IL-1 receptor family. Two forms of the protein exist, a soluble form known as ST2 and a membrane anchored form known as ST2L. The membrane form is expressed by Th2 cells and bone marrow derived mast cells, whereas the soluble form is expressed by serum-stimulated fibroblasts. It binds IL-33 and is structurally similar to IL-1R1. Type 2 innate lymphoid cells (ILC2), originally referred to as natural helper (NH) cells, are a novel target of IL-33. Lung ILC2s participate in the induction of airway inflammation in influenza virus-infected mice and papain-administrated mice. Soluble ST2 plays a role in protecting ILC2 from IL-33 stimulation and thereby maintaining them in a naive state and might be important for the regulation of several disease. Blocking with anti-ST2 antibodies has been shown to alleviate experimental arthritis and airway inflammation. The IL-33-ST2 axis is is involved across a range of diseases including asthma, allergies, obesity and cardiac disease.