8 VACCINE AND SEEUM THERAPY 



(4) black-leg aggressin (used for immunising calves 

 against black-leg). 



Passive Immunity. — In active immunity we have 

 scon that such a condition was acquired by the 

 animal by virtue of its own physiological activities, 

 and it is self-evident that a method of this kind can, 

 in the treatment of disease, be employed prophylac- 

 tically only against possible infection, or in localised 

 acute infections, or in disease which has a long 

 period of incubation. A more hopeful method of 

 treating some diseases is by the use of the serum 

 of animals actively hyperimmunised against the 

 specific disease. Thus by the use of antitetanic 

 serum we can protect animals against the disease. 

 The animal thus protected obviously had taken no 

 active part in its own defence, but was protected from 

 the action of the poison by the substances transferred 

 to it in the serum of the actively immunised animal. 

 Such protection is therefore a purely passive pheno- 

 menon so far as the treated animal is concerned, and 

 the process is for this reason spoken of as " passive 

 immunisation." 



It is mostly in those diseases whose causative 

 agents produce true secretory poisons that the produc- 

 tion of passive immunity can be used with advantage ; 

 this is well exemplified in diphtheria and tetanus, 

 by the use of their respective antiserum. Micro- 

 organisms, however, which exert their harmful action 

 rather by the contents of the bacterial cells than by 

 secreted soluble toxins, do not, so far as is known, 



