Inhibitor of cysteine proteinases. Inhibits host immune responses via its inhibition of host cathepsins (PubMed:19494265). Contributes to the suppression of the host's immune response to tick salivary proteins and is important for successful feeding on hosts (PubMed:17698852). Inhibits differentiation of host dendritic cells (PubMed:19494265, PubMed:25975355). Inhibits proliferation of host T-cells in response to antigen stimulus (PubMed:19494265). Down-regulates TLR2-mediated host responses to infection by B.burgdorferi and the production of the chemokine CCL3 by host dendritic cells (PubMed:25975355). Down-regulates host responses to infection by B.burgdorferi and the production of IFNB1 by host dendritic cells (PubMed:25975355). Down-regulates IL1B production by host mast cells, and this then leads to impaired activation of IL1R1, resulting in decreased IL9 production (PubMed:26078269). Inhibits host inflammatory reactions and recruitment of host neutrophils (PubMed:16772304). Inhibits papain and cathepsin-L (CTSL) (in vitro) (PubMed:16772304, PubMed:17698852, PubMed:20545851). Inhibits cathepsin-S (CTSS) (in vitro) (PubMed:17698852, PubMed:20545851). Inhibits CTSV and CTSC, but to a lesser degree (in vitro) (PubMed:16772304, PubMed:17698852). Monomer. Can form homodimers in vitro, but probably not in vivo. Homodimers are predicted to be inactive; dimerization disrupts the interaction with target proteases. Detected in saliva (at protein level) (PubMed:16772304). Detected in salivary gland and midgut (PubMed:17698852). Combined, RNAi-mediated down-regulation of Salivary cystatin-L and Salivary cystatin-L2 in salivary glands strongly impairs the tick's ability to feed on hosts. About 40% of the ticks cannot feed and die. The remainder show much reduced blood intake, appear unhealthy and display strongly reduced egg laying. RNAi-treated ticks show an impaired ability to suppress the host's immune response to tick salivary proteins. Belongs to the cystatin family.