590 IMMUNITY 



■ microcytase, contained within the polymorphonuclear leucocytes, 

 which has a special digestive action on bacteria. It is the 

 microcytase which gives blood serum its bactericidal properties. 

 It appears to us, however, that Metchnikoff went too far in 

 distinguishing the activities of the two classes of cells so sharply 

 as he did. 



When the properties of antibacterial sera, as above described, 

 are considered in relation to phagocytosis, Metchnikoff gave the 

 following explanation. He admitted that the immune-body is 

 fixed by the bacteria (or red corpuscles, as the case may be), 

 though he did not state that a chemical combination takes 

 place; hence he called it a fixative (fixateur). The immune-bodies 

 are to be regarded as auxiliary ferments (ferments adjuvants) 

 which aid the action of the alexin. Unlike the latter, however, 

 they are formed in excess during immunisation and set free in 

 the serum. He compared 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, maintained 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 mononuclear leucocytes are probably 

 the source of the immune-bodies concerned in hEemolysis, 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. 



Metchnikoff 's work has less direct bearing on the production of 

 antitoxins. He admitted the fixation of the toxin by the anti- 

 toxin to form a neutral compound, and he apparently considered 

 that leucocytes may also be concerned in the production of anti- 

 toxins. Apart, however, from antitoxin formation, he considered 

 the acquired resistance of the cells themselves of high importance 

 in toxin immunity. 



When we consider Metchnikoff's theory as thus extended to 



