In higher animals, the digestion of food is due to several digestive ferments (rennet, pepsin, trypsin, enterokinase, etc.) produced by divers organs (stomach, pancreas, intestines). The phagocytes also manufacture several digestive ferments; their principal digestive juice is a soluble ferment of the trypsin category, to which Metchnikoff gave the name of _cytase_.[25] [25] It is also called _alexine_ or _complement_ by other writers. To the morphological difference of the phagocytes corresponds also a difference in the properties of their cytases, which are suited to the digestion of this or that food. The cytases are kept within the interior of the cells and only escape into the humors when the phagocytes are damaged (Pfeiffer's phenomenon). This kind of ferment does not withstand a temperature above 55° to 58° C. In natural immunity, it plays the principal part by digesting morbid agents inside the phagocytes like any other food. But, in artificial immunity, other soluble ferments come into play, developed in consequence of vaccination. The principal of those is the _fixator_.[26] It is less sensitive than cytasis to high temperatures and can bear a temperature of 65° to 68° C. It is incapable, by itself, of killing and digesting, but by _fixing_ on them, it _bites_ them, so to speak, and makes them sensitive to the action of the phagocytic cytases, which can thus digest them more easily. [26] Designated by other writers by various synonyms: preventive, or sensibilising substance, immunising body, amboceptor. The _fixator_ may be compared to _enterokinase_, a special ferment in the small intestine of higher animals which also does not by itself digest food but which activates in a high degree the digestive power of pancreatic ferments. However, it has the property of fixing itself on fibrin; it is obvious that enterokinase and the fixator have the same essential properties. This similarity again proves that the destruction of morbid agents by the phagocytes really corresponds with actual digestion. It is in consequence of the digestion of vaccinal products that the phagocytes manufacture the _fixator_. Created at the expense of a given vaccinal substance, the _fixator_ has a specific character which corresponds with that substance, whereas the cytase already existing within the phagocytes never has a specific character.