IMMUNITY 79 



Immunity Through Inoculations of Small Doses of Very 

 Virulent Microorganisms. A graduated resistance to the 

 disease is reached somewhat after nature's method. By succes- 

 sive inoculations with increased doses of the virus an immunity 

 is often reached sufficient to withstand ten times the lethal 

 dose. A poison-habit is thus acquired. 



Increased virulence is produced as follows: The cultures 

 may be greatly increased in virulence by successive cultivation 

 through animals, and gradually changing from smaller animals 

 to larger, until an amount of the culture that, at the outset, 

 would not destroy a guinea-pig, becomes finally virulent for 

 chickens and dogs. 



Immunity Through Injections of the Sterilized Products 

 of Bacteria. Cultures sterilized by heat or filtration through 

 germ-filters still contain the chemical products of bacteria 

 the toxins; and when these are injected in gradually increased 

 doses, the same immunity is obtained as with the bacteria 

 themselves. 



Passive Immunity. The blood-serum and tissues generally 

 of animals rendered immune in the ways described above, when 

 injected into susceptible animals, render them immune against 

 the same infection. This has been called passive immunity, but 

 there is no strong reason why this term should be used. The 

 blood-serum of immune animals is simply another means for 

 immunization. It is less permanent than the other forms of 

 immunization, but it appears very soon after the injection, and 

 in a modified form has a curative action even when the symp- 

 toms of the infection are already present in the system. 



Inherited Immunity. An immunity to disease acquired dur- 

 ing the lifetime of the parents is probably never transmitted 

 to the offspring, though the mother may transmit a temporary 

 immunity to the child in utero, or the child itself may have 

 been subjected to the infection at the same time with its mother. 

 But this cannot be called inherited. 



Theories of Immunity. Several older theories only need 

 to be mentioned, as they are no longer tenable. They are 

 the exhaustion theory of Pasteur, the retention theory, and the 



