Serine protease inhibitor that functions in the melanization-mediated immune response (PubMed:16113656, PubMed:20953892, PubMed:33520733, PubMed:33045027, PubMed:21465556, PubMed:25525260). By preventing the activation of phenoloxidases through the inhibiting of serine proteases CLIPB9 and CLIPB10, negatively regulates melanization in the hemolymph (PubMed:16113656, PubMed:20953892, PubMed:33520733, PubMed:33045027, PubMed:21465556, PubMed:25525260). By preventing melanization, has a detrimental role during P.berghei parasite mediated-infection and invasion of the mosquito midgut (PubMed:16113656). Forms a covalent heterodimer with protease CLIPB9; the interaction inhibits CLIPB9 protease activity (PubMed:20953892, PubMed:25525260). Forms a covalent heterodimer with protease CLIPB10; the interaction inhibits CLIPB10 catalytic activity (PubMed:33520733). Secreted into the hemolymph. Expressed in embryos, larvae, pupae, and female and male adults. Protease CLIPB9 binds to SRPN2 via the hinge region resulting in the cleavage of the reactive bond (PubMed:25525260). This leads to a conformational change in SRPN2 which traps CLIPB9 and distorts its active site, resulting in CLIPB9 inactivation (PubMed:25525260). RNAi-mediated knockdown in female causes spontaneous melanization leading to the formation of melanotic pseudotumors and a shortening of lifespan; spontaneous melanization is reduced in a SRPN2 and CLIPB9 or SRPN2 and CLIPB10 mutant backgrounds (PubMed:16113656, PubMed:20953892, PubMed:33520733). During the mosquito midgut infection by the rodent malaria parasite P.berghei, severely reduces oocyst numbers due to an increase in ookinete lysis and melanization (PubMed:16113656). Does not affect ookinete formation (PubMed:16113656). Causes proteolytic cleavage of CLIPA28 and CLIPC9 in absence of E.coli infection (PubMed:33045027). Belongs to the serpin family.