112 BACTERIOLOGY. 



cleat power to prevent infection. Through serums, 

 therefore, we can immunize against an infection, and 

 even stop one just commencing; but as yet we cannot 

 cure an infection which is already fully developed, 

 though even here there is reason to believe that we 

 may possibly prevent an invasion of the general system 

 from a diseased organ as by the pneumococcus from 

 an infected lung in pneumonia. On the whole, the 

 serums which simply inhibit the growth of bacteria 

 have not given, as observed in practice, conclusive 

 evidence of great value in already developed disease. 

 This is partly due to the difficulty to be discussed fully 

 later of determining early enough the exact nature of 

 the bacteria causing the infection. 



Acquired Immunity to Poison. Although the serum 

 of animals which have been infected with any one of 

 many varieties of bacteria is usually both antitoxic and 

 bactericidal, still one of these protective substances may 

 be present almost alone; thus antitoxic substances are 

 present almost exclusively in animals injected with two 

 species of bacteria which produce powerful specific 

 poisons viz., the bacilli of diphtheria and tetanus. 

 When the toxins of either of these are injected in 

 small amounts the animals after complete recovery 

 are able to bear a larger dose without deleterious 

 effects, and these doses in the more suitable animals 

 can be gradually increased until a thousand times a 

 previously fatal dose may be administered without any 

 serious results whatever. To Behring and Kitasato we 

 owe the discovery that this protecting substance accu- 

 mulates to such an extent in the blood that very small 

 amounts of serum are sufficient to protect other animals 

 from the effects of the toxin. 



