IMMUNITY. 177 



account of its size, and, moreover, the horse is very susceptible. 

 Insusceptible animals cannot be made to yield antitoxin, at 

 least of any appreciable strength. 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 The blood of the animal is withdrawn, taking care 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, and the serum allowed to separate in 

 the refrigerator. The serum of the blood is drawn off and 

 constitutes the antitoxin. The use of antitoxin has been em- 

 inently successful and revolutionized the treatment of diphtheria; 

 and it has given partial success in tetanus with an antitoxin 

 prepared by injecting horses with increasing amounts of tetanus 

 toxin. (See the description of the bacteria of these diseases.) 



Ehrhch 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. Ehr- 

 lich also found that the milk of animals which had been 

 immunized with increasing doses of abrin and ricin confers 

 immunity upon suckUngs. In most cases we look to the blood- 

 serum for the immunizing agent. 



There is little, if any, analogy between the tolerance acquired 

 in this manner from bacterial and other toxins and that which 

 victims of the morphine and cocaine habits have for immense 

 doses of these drugs, for no bodies resembhng antitoxins are ob- 

 tained from animals that have been accustomed to such drugs. 



Active and Passive Immunity. — ^The kind of immunity 

 which results from the injection of substances from immunized 

 animals is called "passive immunity." Diphtheria and tetanus 

 antitoxins produce passive immunity. "Active immunity" 

 may be brought about in several ways : 



(i) By a spontaneous attack of an infectious disease; (2) 

 by an attack excited artificially through inoculation with small 

 doses of virulent cultures, or (3) by the administration of 

 large doses of attenuated cultures; (4) or by the injection of 



