SUSCEPTIBILITY AND IMMUNITY. 265 



and an animal vaccinated against it and thus completely immune. So, too, 

 non-fatal doses of these toxines produce in animals of the two categories the 

 same febrile and inflammatory reactions. The proof is clear that there is no 

 special destruction of toxines in the refractory animal, and that the ' ' toxicide 

 property," if it exists, is not one whit more developed after vaccination than 

 before. Passing- in review all these counter theories, we see that each of 

 them can only be applied to a certain number of facts ; in some an attenu- 

 ating or even bactericidal influence of the juices is relied upon, in others an 

 anti-inflammatory action, in yet others a toxicide property. Still the pha- 

 gocytic reaction is the only constant in all those cases of immunity and 

 recovery that have as yet been sufficiently studied, and while certain of the 

 factors mentioned (the attenuating and toxicide properties) do not in the 

 least touch upon the continued existence or otherwise of the microorganism, 

 the bactericidal power of the phagocyte puts an end to the parasite itself, and 

 thus at a given moment prevents further manifestation of its virulence, or 

 preserves the animal attacked at a time when the toxicide properties would be 

 found wanting, and the microbe remaining alive would consequently gain 

 the upper hand. 



But while thus placing before you the important part played by the pha- 

 gocytes, I do not wish it to be thought that these cells are unaided in their 

 contest by other defensive means possessed by the organism. This is far 

 from being my view. Thus, in the febrile reaction, we see a puissant auxil- 

 iary very definitely favoring the work of the phagocytes. This febrile re- 

 action has only to be inhibited as was done by M. Pasteur in the anthrax 

 of fowls and animals naturally refractory to the affection succumb to the 

 ravages of the bacilli. It is not possible at the present time to state fully 

 and accurately all these influences which are associated in aiding phago- 

 cytic action, but already we have the right to maintain that, in the prop- 

 erty of its amoeboid cells to include and to destroy microorganisms, the 

 animal body possesses a formidable means of resistance and defence 

 against these infectious agents. 1 



We are disposed to agree with Metschnikoff in his final conclu- 

 sion,,as above stated in italics. But in view of experimental evi- 

 dence, to be referred to later, we cannot accept the so-called Metsch- 

 nikoff theory as a sufficient explanation for the facts relating to 

 natural and acquired immunity in general, and must regard phago- 

 cytosis simply as a factor which, in certain infectious diseases, ap- 

 pears to play an important part in enabling immune animals to resist 

 invasion by pathogenic bacteria. 



Going back to the demonstrated fact that susceptible animals may 

 be made immune by inoculating them with the toxic products pro- 

 duced during the growth of certain pathogenic bacteria, we may 

 suppose either that immunity results from the continued presence of 

 these toxic products in the body of the inoculated animal, or from a 

 tolerance acquired at the time of the inoculation and subsequently 

 retained by transmission from cell to cell, as heretofore suggested. 

 Under the first hypothesis retention theory immunity may be ex- 

 plained as due to a continued tolerance on the part of the cellular ele- 

 ments of the body to the toxic substances introduced and retained ; 

 or to the effect of these retained toxic products in destroying the 

 pathogenic bacteria, or in neutralizing their products when these are 



1 From the British Medical Journal. 



