246 RESISTANCE AND IMMUNITY 



spring, or may be acquired either by an accidental or an experimental 

 infection. In the former instance, the immunity is said to be natural 

 or inherited and, in the latter, acquired. The acquired type is specific 

 in its nature, i.e., it protects solely against a particular kind of toxic 

 agent, and not against others. Thus, it is possible to develop a resist- 

 ance against the bacillus of diphtheria, while at the same time the body 

 remains open to infections by other germs, such as the bacillus of teta- 

 nus or typhoid. Moreover, while the inherited immunity is generally 

 permanent, the acquired type is often only temporary, enabling the same 

 germ to invade that animal a second time. It is true, however, that 

 most of the infectious diseases, such as typhoid, diphtheria, yellow 

 fever, small-pox, scarlet fever, and others, occur as a rule only once in 

 the same individual. Immunity is also characterized as general and 

 local, the former designation implying that the cells of the body as a 

 whole are affected, and the latter, that solely a particular tissue 

 is so favored. For example, it is a well-known fact that the Peyer's 

 patches of the intestine do not offer favorable conditions for the growth 

 of the typhoid bacillus if they have already served as the seat of a 

 proliferation of this kind. 



Active immunity is the resistance which is acquired by an animal 

 in the course of an active immunization. Various methods are 

 practised to render an animal immune in this manner, but all of them 

 purpose to stimulate the tissues so that they take an active part in the 

 development of the resistance. This end may be attained (a) with 

 attenuated cultures, (6) with sublethal doses of virulent bacteria, 

 (c) with dead bacteria, (d) with the products of the bacteria prepared 

 from filtered cultures, and (e) by feeding the dead cultures. 



Passive immunity is the resistance conferred upon an animal by 

 introducing into its system certain immune agents which have been 

 developed in another animal in the course of an active immunization. 

 This procedure, which promises important therapeutic results, dates 

 from the time of Behring (1890), who showed that the sera of 

 animals immunized against the products of the tetanus or diphtheria 

 bacillus, may be introduced into other animals with the result that the 

 recipients are rendered resistant against these particular poisons. 

 This type of immunity, therefore, is conferred upon an animal without 

 it actively participating in this process of forming those elements which 

 are responsible for the resistance. The sera employed in this process 

 are known as antitoxic sera, and are said to contain antitoxins, or bodies 

 which are specifically antagonistic to toxins. 



In illustration of passive immunization might be mentioned the 

 procedure usually followed in protecting human beings against the 

 toxins of the diphtheria bacillus. The antitoxin concerned in this 

 reaction is obtained in larger quantities with the help of young and 

 vigorous horses. The systems of these animals is first accustomed to 

 the diphtheria poison by the subcutaneous administration of small 

 doses of diphtheria toxin. The doses are then gradually increased. 



