66 RELATION OF BACTERIA TO DISEASE 



Acquired immunity is that resistance which a person 

 obtains by passing through an attack of disease. That a 

 second attack of measles or scarlatina or typhoid fever 

 seldom occurs is well known. Such an acquired immunity 

 is called active acquired immunity because the economy has 

 had to work for its own protection, and it is only good for the 

 one kind of disease, supplying no protection to any other kind : 

 that is, it is specific. 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 

 given to the patient. This horse is said to possess active 

 artificial immunity because it has been given 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 antisera against meningitis and pneumonia have been 

 produced. 



Artificial immunity is one that has been produced inten- 

 tionally by the physician. The term may be correctly applied 

 to any form except the natural or active acquired immunities, 

 but it is usually reserved for the various procedures in experi- 

 mental medicine whereby antiserums or vaccines are manu- 

 factured. 



Anti-endotoxins. These bodies, comparable to antitoxins, 

 are developed in the blood serum when the system harbors 



