PHAGOCYTOSIS 43 



same time, it must be remembered that the immunity 

 set up by the use of this class of serum is of the passive 

 kind, and has but a limited duration, lasting on the 

 average about eight to twelve days ; it is therefore 

 essential that, in order to keep an animal immune 

 to this class of disease, it is necessary to give it 

 an injection of the antiserum about every ten 



days. 



Now, when w^e turn to those diseases caused by 

 micro-organisms which do not secrete true soluble 

 toxins, we cannot produce antisera for the bacteria 

 responsible for such diseases, which will contain 

 antitoxic substances; how^ever, such sera will contain 

 immune bodies, in the form of either bactericidal or 

 bacteriolytic substances. Up to the present, nothing 

 but disappointment has persistently followed in the 

 wake of serum therapy as applied to these infectious 

 diseases. This disappointment, I believe, is due to 

 the formation of anti- antibodies which I have already 

 mentioned. 



To give animals immunity against diseases caused 

 by this latter class of micro-organisms, we must 

 make use of vaccines. The administration of bacterial 

 cultures for purposes of prophylaxis is a procedure 

 which is easily explicable on ordinary principles, 

 since the introduction of a dose of virus which the 

 individual is capable of overcoming by means of his 

 natural powers of resistance ensures a supply of 

 antibodies being formed to resist subsequent attacks 

 by the infective organisms, since to confer active 



