100 INFECTION AND IMMUNITY. 



tissues which are known to be the tissues susceptible to the 

 action of the bacterial poison. 



V. By introducing into the animal inert particles, such as 

 carmine, mixed with the bacteria. 



Forced immunization of animals consists in introducing grad- 

 ually and in increasing doses bacterial toxins in sufficient quan- 

 tity to produce a reaction, but in quantity too small to produce 

 deleterious effects. In this way it has been found that ani- 

 mals immunized produce substances in their tissue-fluids which 

 when inoculated in susceptible animals serve to protect them 

 against the deleterious action of those bacteria or their toxins. 



B. Passive Immunity. 



Passive immunity, as mentioned previously, is established 

 immediately, but does not last. It is produced by the injection 

 of material (serum) obtained from animals which have been 

 actively immunized against the bacteria whose toxins it is 

 wished to destroy. The substances in these immune sera pos- 

 sessing the power to neutralize the toxins are called antitox- 

 ins. No active part is taken in this form of immunization by 

 the individual being immunized, and it is, therefore, called 

 passive immunization. 



The Antitoxic and Antimicrobic Blood- serums. 



The blood-serum of animals used for the purpose of protect- 

 ing others is said to be antitoxic, when it has been obtained 

 by the action of the toxins of the bacteria on the animal ; 

 and to be antimicrobic, when it has been obtained by means 

 of the action of virulent or attenuated cultures on those 

 animals. 



Uses. Antitoxic serum is employed chiefly in the toxic dis- 

 eases, such as diphtheria, tetanus, etc. 



Theories in Explanation. A number have been suggested. 

 Some believe that the antitoxin is a chemically changed toxin 

 others claim that it is a sort of enzyme produced by The toxin ; 

 others again state that it is the product of the cytic activity 



