It is commonly agreed that invertebrates lack functional antibodies, but still they can efficiently combat and kill parasites trying to gain entry into the body cavity. Often parasites are melanized in the arthropod haemocoel and thus the enzyme responsible for this melanization, phenoloxidase, has been believed to be involved in host defence. Phenoloxidase is present in arthropods as a zymogen and is activated by microbial polysaccharides in a stepwise process involving proteolytic enzymes. In crustaceans it appears that the proPO‐system upon activation produces factors which are involved in the communication between different blood cells. In this article the role of the proPO‐system in host defence is reviewed as well as some of the properties of individual proPO‐components.