Cystatins are a family of proteins that act as inhibitors of cysteine proteases, which are enzymes that break down proteins. They play a crucial role in regulating various biological processes, including immune responses and cell signaling. Cystatins are found in many organisms, from humans to plants, highlighting their importance in maintaining cellular functions. These proteins are classified into different types, such as cystatin A, cystatin B, and cystatin C, each with specific roles in the body. Abnormal levels of cystatins have been linked to various diseases, including Alzheimer's disease and cancer, making them a focus of research in understanding and treating these conditions.