38 BACTERIOLOGY. 



cation has been in the preventive inoculation against 

 typhoid fever in the army. 



immunity There is another type of immunity that can be 



conferred without the body tissues taking any active 

 part in the process. For this reason it is called passive 

 immunity. In 1890 von Behring discovered that the 

 blood-serum of animals that had been immunized to 

 the poisons of diphtheria and tetanus, if injected into 

 other animals, would protect them also. Quite re- 

 cently Dr. Flexner, at the Rockefeller Institute in New 

 York, made similar observations in connection with 

 the poison of the meningococcus, the organism caus- 

 ing the epidemic form of cerebrospinal meningitis. 



Perhaps a brief description of the way diph- 

 theria antitoxin is made will make this type of im- 

 munity better understoood. The animal used in the 

 commercial preparation of diphtheria antitoxin is the 

 horse. At the start the animal is inoculated with a 

 very small dose of the diphtheria toxin obtained by 

 growing the diphtheria bacillus on large flasks of 

 bouillon. The bacilli are filtered out and the filtrate 

 containing the soluble diphtheria toxin is used for 

 injecting. The effect of the first injection is to make 

 the horse sick, but not fatally so. At the end of a 

 week a second injection is made with the same dose, 

 but the animal is now able to stand the poison without 

 ill effect. Each week the dose is increased until at the 

 end of two or three months the animal is able toi with- 

 stand enormous doses of the poison without ill effect, 

 due to the protective substances formed in its body. 



