Inhibitor of cysteine proteinases. Inhibits host immune responses via its inhibition of host cathepsins. Contributes to the suppression of the host's immune response to tick salivary proteins and is important for successful feeding on hosts. Inhibits differentiation of host dendritic cells. Inhibits proliferation of host T-cells in response to antigen stimulus. Down-regulates TLR2-mediated host responses to infection by B.burgdorferi and the production of the chemokine CCL3 by host dendritic cells. Down-regulates host responses to infection by B.burgdorferi and the production of IFNB1 by host dendritic cells. Down-regulates IL1B production by host mast cells, and this then leads to impaired activation of IL1R1, resulting in decreased IL9 production. Inhibits host inflammatory reactions and recruitment of host neutrophils. Inhibits papain and cathepsin L (CTSL) (in vitro). Inhibits cathepsin S (CTSS) (in vitro). Inhibits CTSV and CTSC, but to a lesser degree (in vitro). Monomer. Can form homodimers in vitro, but probably not in vivo. Homodimers are predicted to be inactive; dimerization disrupts the interaction with target proteases. Belongs to the cystatin family.