Cysteine protease inhibitor which inhibits members of the peptidase C1 family (PubMed:1512269, PubMed:12704112). In the human host, inhibits CTSL/cathepsin L and CTSS/cathepsin S and to a lesser extent CTSB/cathepsin B which may cause defects in antigen processing and thereby impair antigen-driven T cell proliferation (PubMed:12704112). Expressed in the cuticle of L3 and L4 larvae, female adult, and in the eggshell of developing microfilariae. Expressed in infective larvae (L3, L4) and adult stages. Belongs to the cystatin family.