Serine protease which, by cleaving and activating prophenoloxidase (PPO1) after immune challenge, plays an essential role in the melanization immune response to septic wounding (PubMed:16256951, PubMed:16322759, PubMed:19071960, PubMed:24260243, PubMed:16861233). May function in diverse Hayan-dependent PPO1-activating cascades that are negatively controlled by different serpin proteins; Spn27A in the hemolymph and Spn77BA in the trachea (PubMed:18854145, PubMed:24260243, PubMed:16322759). Important for the innate immune response to fungi (PubMed:16861233). Regulation of melanization and PPO1 activation appears to be largely independent of the Toll signaling pathway (PubMed:16861233). Interacts with Spn27A. First detected in stage 11 embryos, in cells close to the gnathal region. At stage 13, expressed in numerous cells that are dispersed throughout the embryo. During organogenesis predominantly expressed under the cuticle in the proventricular zone close to the hindgut. In L3 larvae, expressed in the lymph glands, the hematopoietic organ that flanks the dorsal vessel and in mature hemolymph crystal cells that appear in clusters attached to diverse organs. Up-regulated after wounding. Levels are higher with septic wounding, using either Gram-negative or Gram-positive bacteria. The clip domain consists of 35-55 residues which are 'knitted' together usually by 3 conserved disulfide bonds forming a clip-like compact structure. RNAi-mediated knockdown is semi-pupal lethal, with 50% of pupae dying at the late pupal stage or during eclosion (PubMed:16861233). Adults display impaired melanization at the site of septic infection (septic injury) using either Gram-negative or Gram-positive bacteria (PubMed:16256951, PubMed:16861233, PubMed:19071960). Reduced survival, PPO1 activity and induction of the antimicrobial peptide Drs following septic injury using the fungus B.bassiana (PubMed:16256951, PubMed:16861233). Septic injury with various bacteria reduces survival (L.monocytogenes, S.typhimurium and S.aureus) and in some cases decreases (E.faecelis) or increases bacterial growth (L.monocytogenes, S.typhimurium and B.cepacia). Aseptic injury reduces survival (PubMed:19071960). Aseptic wounding has no effect on survival (PubMed:22227521). Significant increase in survival after immune challenge with S.pneumoniae although there is no increase in melanization (PubMed:19071960). No effect on the survival following infection with various bacteria (E.coli, B.cepacia and E.faecelis) (PubMed:19071960). No induction of the antimicrobial peptides Drs and Dpt following infection with E.carotovora (PubMed:16861233). Belongs to the peptidase S1 family. CLIP subfamily.