252 A MANUAL OF BACTERIOLOGY 



(6) By injection of the blood-serum derived from an 

 animal treated or immunised by method (3) that is to 

 say, antitoxins or other anti-bodies (e.g. amboceptors) 

 are introduced. 



The immunity acquired by methods (l)-(4) is known 

 as " active immunity," because the animal's cells and 

 tissues are altered by the process, so that they are no 

 longer susceptible to the microbe or its toxin. The 

 immunity conveyed by method (6) the injection of an 

 immune serum is known as " passive immunity," because 

 the immunity lasts only so long as the anti-bodies 

 remain ; there is no active participation of the animal's 

 cells and tissues in the process. Active immunity is 

 generally of long diiration some months at least and 

 is not transmissible to the fetus ; but passive immu- 

 nity is of short duration two to four weeks and is 

 transmissible to the fetus and nursling. Acquired 

 immunity to toxins may be due to the elimination 

 of the receptors concerned in the fixation of the toxin 

 by the cells, or to the production of antitoxin. The 

 leucocytes are probably the active agents in destroying 

 and eliminating toxin, whether neutralised by antitoxin 

 or not. 



Various explanations have been given of the production 

 of acquired immunity against the organisms. Pasteur 

 suggested that the organism, by its growth in the body, 

 exhausts some specific pabulum necessary for its develop- 

 ment, so that it cannot again grow in the animal which 

 has been attacked. This hypothesis, therefore, pre- 

 supposes that in the body there is some nutrient material 

 necessary for the growth of each species, which is difficult 

 to believe, and is negatived by the fact that an organism 

 will grow in the blood and tissues removed from an animal 

 vaccinated against, and insusceptible to, the disease 

 produced by itself. 



