Cystatin is a type of protein that plays a crucial role in regulating the activity of enzymes called cysteine proteases. These enzymes are involved in various biological processes, including protein degradation and immune responses. Cystatin helps maintain a balance in these processes by inhibiting the activity of cysteine proteases, thus protecting tissues from excessive breakdown. There are several types of cystatins, including Cystatin A, Cystatin B, and Cystatin C, each with specific functions in the body. Cystatin C is particularly important in kidney function and is often used as a biomarker for assessing kidney health. Overall, cystatins are essential for maintaining cellular homeostasis and protecting against tissue damage.