Serine protease inhibitor that functions in embryonic dorsoventral patterning and the melanization immune response (PubMed:14654000, PubMed:14667416, PubMed:12408809, PubMed:12456640, PubMed:16861233, PubMed:16322759, PubMed:18801354, PubMed:22227521, PubMed:24260243, PubMed:24788090). Regulates dorsoventral axis formation during early development by inhibiting the serine protease easter, and is therefore important for restricting activity of the Toll signaling pathway to the ventral part of the embryo (PubMed:14654000, PubMed:14667416). Also plays an essential role in the melanization immune response to both physical wounding and septic infection using certain bacteria and fungi (PubMed:12408809, PubMed:12456640, PubMed:18801354, PubMed:22227521). Negatively regulates the Hayan-dependent prophenoloxidase 1 (PPO1)-activating cascade in the hemolymph by inhibiting the serine proteases MP1 and Sp7 (PubMed:12408809, PubMed:12456640, PubMed:16861233, PubMed:16322759, PubMed:18801354, PubMed:22227521, PubMed:24260243, PubMed:24788090). May be involved in negatively regulating the melanotic encapsulation around eggs of the parasite L. boulardi (PubMed:12408809, PubMed:15749104). Interacts with Sp7. Secreted into the hemolymph (PubMed:12456640). Detected in the perivitelline fluid of embryos (PubMed:14667416). Expressed at all developmental stages, with lowest levels at the adult stage (at protein level) (PubMed:12408809). Uniformly expressed along the dorsoventral axis of embryos (at protein level) (PubMed:14667416). At the periphery of syncytial blastoderm embryos, expression is often weakest at the anterior and posterior termini (at protein level) (PubMed:14667416). In adults, up-regulated between 0 and 3 hours after bacterial infection and fungal infection (PubMed:12408809). Then appears to be down-regulated in the hemolymph between 3 and 17 hours after bacterial infection (PubMed:12456640, PubMed:16322759). Loss of maternal expression causes arrest prior to the syncytial blastoderm stage (PubMed:14667416). A percentage of larvae and adults, 40% and 35% respectively, display spontaneous melanization (PubMed:12408809). Melanization occurs mainly around the internal organs such as the gut and fat body, but is not associated with the barrier epithelia of the body wall (PubMed:12408809). Increased melanization following wounding coupled with bacterial infection (PubMed:12408809). Phenoloxidase (PO) activity is increased in the hemolymph, both in mutants that display melanization or those that do not (PubMed:12408809). RNAi-mediated knockdown results in spontaneous melanization (PubMed:22227521). After septic injury PPO1 is absent in the hemolymph, whereas its active form PO is present but at a reduced level (PubMed:18801354). Belongs to the serpin family.