IMMUNITY. l8l 



are cultivated in bouillon. The cultures are freed from all 

 living bacilli by filtration. The liquid filtrate contains the 

 toxin. This filtrate is injected into healthy animals, usually 

 horses, in increasing doses. Eventually enormous doses of 

 toxin are given, and the animal acquires a high degree of 

 immunity. The blood of the animal is withdrawn, taking 

 care to avoid contamination. The serum of the blood is 

 collected and constitutes the antitoxin. Such antitoxins 

 have produced brilliant results in the treatment of diph- 

 theria, and have given partial success with tetanus. " (See 

 the articles on the bacteria of these diseases.) 



Ehrlich discovered that the vegetable toxins, abrin and 

 ricin, behave in a manner very similar to soluble bacterial 

 poisons when injected into animals, and that by their injec- 

 tion an immunity for the same poisons may be secured. 

 There is an analogy between the tolerance acquired in this 

 manner from bacterial and other toxins and that which vic- 

 tims of the morphine and cocaine habits have for immense 

 doses of those drugs. Ehrlich also found that the milk of 

 animals which had been immunized against abrin and ricin 

 might confer immunity upon young animals. In most cases 

 we look to the blood-serum for the immunizing agent. 



Active and Passive Immunity. The kind of immunity 

 which results from the injection of substances from im- 

 munized animals is called " passive immunity." Diphtheria 

 and tetanus antitoxins produce passive immunity. " Active 

 immunity " may be brought about in several ways : 



(i) By an ordinary attack of an infectious disease; (2) 

 by an attack excited artificially through inoculation with 

 small doses of virulent cultures, or (3) large doses of atten- 

 uated cultures; (4) or by the injection of bacterial products 

 (toxins) freed from the bacteria themselves. Pasteur's 

 methods of protective inoculation for anthrax, etc., and 

 Haffkine's injections for bubonic plague produce active 



