Cystatins are a family of proteins that function 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, cell signaling, and tissue remodeling. Cystatins are found in many organisms, including humans, and are important for maintaining cellular homeostasis. These proteins are classified into three main types: cystatin A, cystatin B, and cystatin C. Each type has distinct functions and tissue distributions. Abnormal levels of cystatins have been linked to various diseases, including Alzheimer's disease and cancer, highlighting their significance in health and disease management.