206 A MANUAL OF BACTERIOLOGY 



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 duration some months at least and is not trans- 

 missible to the fetus ; but passive immunity 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 

 the neutralising 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. 



Pasteur's '" exhaustion " theory has been revived by Ehrlich 1 

 in a modified form, under the name of " atrepsy," to explain certain 

 cases of immunity. Thus, for a chemical poison to act, Ehrlich 

 assumes that particular receptors in the protoplasm for binding 

 the poison are necessary ; these he terms " chemo -receptors." 

 Bird-pox, virulent for both fowl and pigeon, if passed through the 

 pigeon becomes completely avirulent for the fowl. To explain this 

 Ehrlich suggests that the parasite in passing through the pigeon 

 has to assimilate substances different from those assimilated during 

 its passage through the fowl ; therefore that part of the receptors 

 which deals with the nutritive substances of the fowl's organism is 



1 " Harben Lecture," ii, Journ. Roy. Inst. Public Health, 1907. 



