Cystatin C is a small protein produced by all human cells, primarily found in the blood and other bodily fluids. It plays a crucial role in regulating the activity of enzymes that break down proteins, helping to maintain a balance in the body's immune response and tissue repair. Cystatin C is often used as a biomarker to assess kidney function. Unlike creatinine, another common marker, cystatin C levels are less affected by factors like age, gender, and muscle mass, making it a more reliable indicator of kidney health in various populations.