Cynomolgus Monkey (Macaca fascicularis) Interleukin-1 Receptor Antagonist (IL-1Ra) is an anti-inflammatory cytokine produced by monocytes, macrophages, neutrophils, and other immune cells that serves as a critical regulator of IL-1–mediated inflammation. IL-1Ra competitively binds to the IL-1 receptor type 1 (IL-1R1) without initiating intracellular signaling, thereby blocking the pro-inflammatory effects of IL-1α and IL-1β and limiting activation of NF-κB and MAPK pathways that drive fever, leukocyte recruitment, endothelial activation, and systemic inflammatory responses. Highly homologous to human IL-1Ra, the cynomolgus monkey protein plays a key role in maintaining cytokine balance in nonhuman primate models of infectious diseases such as dengue, influenza, SARS-CoV-2, tuberculosis, and bacterial sepsis, as well as in models of autoimmune and inflammatory disorders. The IL-1/IL-1Ra balance is particularly important in studies evaluating cytokine release syndromes and immunotoxicology of biologics. Because cynomolgus monkeys are widely used in preclinical safety testing, vaccine development, and translational immunology research, characterization of IL-1Ra is essential for understanding inflammasome regulation, inflammatory resolution, and therapeutic modulation of IL-1 signaling, providing a critical bridge between nonhuman primate models and human inflammatory disease research.