38 
BACTERIOLOGY. 
Passive 
immunity 
cation has been in the preventive inoculation against 
typhoid fever in the army. 
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. Later Dr. 
Flexner, at the Rockefeller Institute in New York, 
made similar observations in connection with the 
poison of the meningococcus, the organism causing 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 understood. 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 to with¬ 
stand enormous doses of the poison without ill effect, 
due to the protective substances formed in its body. 
