IL-1 receptor–like 1 (IL1RL1) is a membrane receptor involved in TH2 inflammatory responses and eosinophilia. It has previously been described that levels of the interleukin-1 like 1 (IL1RL1) protein can be used to diagnose cardiovascular disease and determine the prognosis for a patient with cardiovascular disease. The ligand for IL1RL1 has been described and named IL-33. Mutants in IL1RL1 have been associated with blood eosinophil counts in a genome-wide association study and with asthma in family-based and case-control studies. As an important mediator involved in many immune and inflammatory responses, this cytokine has been implicated as a regulator of both the development and effector phases of type 2 helper T cell responses, and as a negative feedback modulator of macrophage proinflammatory function. IL33 is a specific ligand of ST2L and induces the production of Th2 cytokines.