A major innate defense system in invertebrates is the melanization of pathogens and damaged tissues. This important process is controlled by the enzyme phenoloxidase (PO). The conversion of prophenoloxidase to the active form of the enzyme can be brought about by minuscule amounts of molecules such as lipopolysaccharide, peptidoglycan and beta-1,3-glucans from microorganisms.
However it still has many arguments in it innate immune function, especially in model invertebrate animal.  The proPO homologous-protein in mammal also does not have any immune activity. Thus, it might be difficult to conclude its function in immunity.
- Beck, Gregory; Habicht, Gail S. (November 1996). "Immunity and the Invertebrates" (PDF). Scientific American 275 (5): 60–66. doi:10.1038/scientificamerican1196-60.
- Cerenius, L; Söderhäll, K (2004). "The prophenoloxidase-activating system in invertebrates". Immunological reviews 198: 116–26. doi:10.1111/j.0105-2896.2004.00116.x. PMID 15199959.
- Söderhäll, K; Cerenius, L (1998). "Role of the prophenoloxidase-activating system in invertebrate immunity". Current opinion in immunology 10 (1): 23–8. doi:10.1016/S0952-7915(98)80026-5. PMID 9523106.
- Leclerc, V; Reichhart, JM (2006). "Prophenoloxidase activation is not required for survival to microbial infections in Drosophila". EMBO Rep 7 (2): 231–5. doi:10.1038/sj.embor.7400592. PMID 1369246.
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