256 ANALOGY WITH DIGESTION 



avidity, since they are already partially digested. We should 

 explain the phenomena very differently : the haemolysis is a result 

 of the action of amboceptor and complement, and the phagocytosis 

 of the action of an opsonin. 



The explanation of bacteriolysis and haemolysis by means of 

 complement and amboceptor might appear to be difficult on the 

 theory of the reference of the whole process to the digestive 

 action of the phagocytes, but Metchnikoff has applied the 

 researches of Pawlow in a very ingenious way to show a 

 parallelism between cytolysis and digestion. It will be remem- 

 bered that pancreatic digestion depends upon the action of two 

 substances an enzyme, protease, which occurs in the pancreatic 

 juice, and another substance, enterokinase, which occurs in the 

 succus entericus. Metchnikoff regards the protease as analoerous 



0>wJrV--tX>ft&tr^ 



with cytase, and the enterokinase as analogous with compleitieftt 

 or substance sensibilatrice. Delezenne showed (though I believe 

 his results are not universally accepted by physiologists) that 

 protease has no power of attaching itself to proteids, whereas 

 enterokinase has such a power, and the substance thus sensitized 

 can then be attacked by protease. This, if true, is exactly 

 similar to the action of amboceptor and complement. We may 

 suppose, then, that amboceptor represents some substance used 

 by the phagocyte to assist the action of cytase or alexin on the 

 bacteria, etc., and normally retained in the protoplasm. 



When an organism which is easy to deal with is injected it is 

 taken up by the phagocytes and dealt with in their protoplasm, 

 no preparatory action being necessary. Under other circum- 

 stances, when the infection is a more virulent one, some of the 

 phagocytes are killed and dissolved, and their digestive enzymes 

 escape, and partially digest the bacteria, which are then ready for 

 phagocytosis. When there is a balanced contest of long dura- 

 tion another substance is formed, which, under normal circum- 

 stances, is not necessary for intracellular digestion, but which 

 facilitates it in difficult cases ; this also may escape into the 

 juices, and still further facilitate the preparatory stages of diges- 

 tion. Lastly, as a rarity, enough of these soluble substances may 

 be set free to dissolve the bacteria altogether, and render phago- 

 cytosis unnecessary. To Metchnikoff cellular digestion and . 

 nutrition are the important factors in immunity ; extracellular 

 action is a less important and occasional phenomenon, and occurs 

 mainly or entirely as a preparation for phagocytosis. His theory 



