Crustaceans lack immune memory and have therefore to rely on innate immune reactions.
One such reaction is the clotting process which is very efficient and rapid and consists of a clotting protein present in plasma and a transglutaminase in the blood cells. The clotting protein has been cloned and belongs to the vitellogenin superfamily of proteins. Clotting proteins have also been purified from shrimp and they are very similar in properties to that of crayfish. This system has been studied in greatest detail in freshwater crayfish and most of the proPO-components have been purified and cloned. Recently, we have also been able to clone some proPO-componenets from a shrimp, Peneaus monodon.