YKL-40 also known as GP-39, CHI3L1 is a glycoprotein member of the glycosyl hydrolase 18 family. YKL-40 plays a crucial role in the inflammatory response. Elevated levels of YKL-40 in serum and synovial fluid are associated with inflammatory diseases, particularly correlating with the degree of joint destruction in rheumatoid arthritis, making YKL-40 a significant biomarker for disease progression. YKL-40′s structure, characterized by glycosylation and ability to form homodimers, is essential for modulating inflammation and tissue remodeling. YKL-40′s role as a candidate autoantigen in rheumatoid arthritis highlights YKL-40′s importance in the immune response, as YKL-40 aids cells in adapting to environmental changes, thus contributing to the pathophysiology of joint diseases.