The prophenoloxidase activating system constitutes a system for recogn
ition of foreignness in several invertebrates. The system has been esp
ecially well studied in crustaceans and it will now be possible to beg
in structural comparisons between components of the prophenoloxidase a
ctivating system and components of other cascade systems which functio
n in host defense such as the vertebrate complement and blood coagulat
ion.