150 INFECTION AND IMMUNITY. 



exceptions to this tendency, however; perhaps the 

 most striking example is found in the fact that 

 whereas sheep ordinarily are extremely susceptible 

 to anthrax, Algerian sheep are relatively immune. 

 Similarly the white rat is immune (relatively) 

 and the wild rat is susceptible to anthrax. 

 Factors in Natural immunity m&j depend on a lack of 

 immunity, pathogenicity on the part of the organism for the 

 host, which is equivalent to insusceptibility on the 

 part of the host; or, on the presence in the host 

 of a sufficient quantity of antibacterial and anti- 

 toxic substances; or, as suggested by Ehrlich, on 

 the inability of the micro-organism to proliferate 

 in the host because proper nutritive substances are 

 not present, or, if present, are bound to other sub- 

 stances in such a way that they are not available as 

 food for the organisms. 



As jtateoMn the preceding chapter, immunity 

 may be either antibacterial or antitoxic, i. e., the 

 immunity may in one case depend on the power 

 of the animal's tissues and fluids to destroy the 

 micro-organisms, or, in another, on their power 

 to neutralize or destroy the bacterial toxins. 



The distinction between antibacterial and anti- 

 toxic immunity is demonstrated more readily in 

 acquired than in natural immunity. When one 

 has recovered from typhoid fever, for example, his 

 serum has acquired an increased power of killing 

 the typhoid bacillus, while at the same time it 

 appears to have little or no power of neutralizing 

 the poisons of this organism. Acquired immunity 

 to diphtheria, on the other hand, is characterized 

 by the power of the serum to neutralize the toxin 

 of the diphtheria bacillus, although it hardly 



