Interleukin 1 receptor-like 1 (IL1RL1), also known as ST2, DER4, FIT-1, or IL33R, belongs to the interleukin-1 receptor family. Its transmembrane form ST2L contains three immunoglobulin-like C2-type domains and one TIR domain. IL1RL1 (ST2L) serves as the ligand-binding subunit of the receptor for interleukin-33 (IL-33), which also requires IL-1R accessory protein (IL-1RAcP) for signaling. Upon IL-33 stimulation, the receptor complex recruits MYD88, IRAK1, IRAK4, and TRAF6, leading to the phosphorylation of MAPK3/ERK1, MAPK1/ERK2, MAPK14 (p38), and MAPK8 (JNK). IL1RL1 is involved in Th2-type helper T-cell function. The intracellular TIR domain of IL1RL1 can interact with MYD88, IRAK1, IRAK4, and TRAF6.