THE THEORY OF PHAGOCYTOSIS 591 



the serum. He compares their action to that of enterokinase, 

 a ferment which is produced in the intestine and which aids 

 the action of trypsin. Thus, when the bacteria have fixed the 

 immune-body, their digestion is facilitated either within the 

 phagocytes, or outside of them when the alexin has been set 

 free by phagolysis. He, however, maintains that extracellular 

 digestion or lysogenesis does not take place without the 

 occurrence of phagolysis. The source of immune-bodies is, in 

 all probability, also the leucocytes, as these substances are 

 specially abundant in organs rich in such cells spleen, lymphatic 

 glands, etc. ; here again the mono-nuclear leucocytes are probably 

 the source of the immune-bodies concerned in haemolysis, the 

 polymorpho-nuclear leucocytes the source of those concerned 

 in bacteriolysis. Although the immune-bodies are usually 

 set free in the serum, this is not always the case; sometimes 

 they are contained in the cells, and this probably occurs when 

 there is a high degree of active immunity against bacteria 

 without a serum having an antibacterial action, the powers of 

 intracellular digestion being in such cases increased. In this 

 way the facts of immunity can be explained so far as these 

 concern the destruction of bacteria. 



MetchnikofFs work has less direct bearing on the production of 

 antitoxins. He admits the fixation of the toxin by the antitoxin 

 to form a neutral compound, and he apparently considers that 

 leucocytes may also be concerned in the production of antitoxins. 

 Apart, however, from antitoxin formation, he considers the 

 acquired resistance of the cells themselves of high importance 

 in toxin immunity. 



When we consider Metchnikoff's theory as thus extended to 

 cover recently established facts, it must be admitted that it 

 affords a rational explanation of a considerable part of the 

 subject, though the elucidation of the chemiotactic phenomena 

 during immunisation as explained above detracts from the im- 

 portance which he attached to the leucocyte. It, however, does 

 not afford explanation of the multiplicity and specificity of 

 antitoxins as Ehrlich's does; on the other hand, it is more 

 concerned with the cells of the body as destroyers or digesters of 

 bacteria. As regards the subject of antibacterial sera, the results 

 of these two workers may be said to be in harmony in some of 

 the fundamental conceptions. And it is of interest to note 

 that Metchnikoff, starting with the phenomena of intracellular 

 digestion, has arrived at the giving off of specific ferments by 

 phagocytes ; whilst Ehrlich, from his first investigations on the 

 constitution of toxins, has arrived at an explanation of antitoxins 



