IMMUNITY 65 



fever. Such an acquired immunity is called active 

 acquired immunity because the economy has had to 

 work for its protection. There is also a passive acquired 

 immunity, by which is meant that some protective 

 substances from another individual are added to the 

 natural resistance of the body. This passive acquired 

 immunity is very well shown in diphtheria when the 

 serum of a horse which has been rendered resistant 

 to the toxin of the diphtheria bacilli is introduced. 

 This horse is said to possess active artificial immunity 

 because it has received the poisons themselves in such 

 a manner that its blood has been able to develop anti- 

 or against-poisons or antitoxins, strong enough to 

 neutralize the toxins of the diphtheria bacilli. This 

 blood is suitable to be transferred to another individual, 

 and in the body of the latter offsets the effects of the 

 toxin of the diphtheria bacillus. In other words, the 

 horse's economy has worked actively against the poison, 

 whereas the person receiving the horse's serum has not 

 worked, but merely received a neutralizing substance 

 from the horse's serum; it has been passive. This 

 passive immunity is also seen in the treatment of 

 tetanus by an antiserum (see antitoxins) and lately 

 Flexner has elaborated a method by which the poisons 

 of the meningitis coccus are neutralized, here again 

 by using the serum of horses injected with this coccus. 

 Artificial immunity is one that has been produced 

 intentionally by the physician. It may be correctly 

 applied to any form except the natural or active 

 acquired immunities, but it is usually reserved for the 

 various procedures in experimental medicine whereby 

 antiserums or vaccines are manufactured. 

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