Inhibitor of cysteine proteinases. Inhibits host immune responses, probably 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 (PubMed:17698852). Down-regulates TLR2-mediated host responses to infection by B.burgdorferi and the production of chemokines CCL3 and CXCL10 by host dendritic cells (PubMed:25975355). Enhances infection by the tick-transmitted pathogen B.burgdorferi (in vitro) (PubMed:20545851). Inhibits host inflammatory responses to A.phagocytophilum infection (PubMed:24686067). Inhibits papain (in vitro) (PubMed:20545851). Inhibits cathepsin-L (CTSL) (in vitro) (PubMed:17698852, PubMed:20545851). Inhibits cathepsin-L2 (CTSV) (in vitro) (PubMed:17698852). Monomer. Detected in salivary gland and midgut. Strongly up-regulated in salivary gland during feeding. 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.