Cystatins in Immune System Abstract Lysosomal cysteine proteases, the cathepsins, classified as clan C1, 1 were long believed to be responsible for the terminal protein degradation in the lysosomes. This view has changed since they are involved in a number of important cellular processes, such as antigen presentation, 2 bone resorption, 3 apoptosis 4 and protein processing, 5 as well as several pathologies