IL-1R1 (IL-1 receptor type 1, also known as p80, IL-1R-alpha or CD121a) is a 80 kDa transmembrane glycoprotein and a member of the interleukin-1 receptor family (PMID: 35163653; 29248002). It is a receptor for IL-1α, IL-1β, and interleukin-1 receptor antagonist (IL-1Ra). IL-1α and IL-1β interact with the extracellular domain of IL-1R1, triggering the recruitment of an accessory receptor, the IL-1RAcP, resulting in a functional receptor complex that initiates IL-1R1 signaling cascades (PMID: 35163653). IL-1Ra competes with active IL-1 and blocks binding to their common activating receptor IL-1R1 and, thus, inhibits IL-1 signaling (PMID: 35332327; 11641511). It has been reported that IL-1R1 may also bind IL-38 (PMID: 29248002). IL-1R1 expression has been found in many cell types including T cells, macrophages, neutrophils, eosinophils, basophils, mast cells, fibroblasts, and endothelial cells.