INFECTION AND IMMUNITY. 567 



MODE OF ACTION OF PATHOGENIC BACTERIA. 

 The development of our knowledge of immunity began 

 with the recognition of the relation existing between the 

 toxins and antitoxins. It was found that on the injection 

 into animals of a dose of toxin certain reactions occurred 

 varying with the size of the dose that is, if the dose 

 was a fatal one the animal died within a definite period 

 of time, which one may call the incubation period. This 

 incubation period is shorter the greater the amount of 

 toxin injected. If the amount of toxin injected is less 

 than the minimum fatal dose the animal shows after a 

 time certain reactions, and after recovery from the 

 effects of the injection larger doses of the toxin can be 

 injected without destroying the animal. At the same 

 time there appears in the serum an antibody which when 

 injected into a normal animal will protect it against the 

 minimum fatal dose of the toxin. Of great importance 

 in the relation between the toxin and antitoxin is the law 

 of multiple proportions, whereby one understands a 

 definite relation between the two that is, a definite 

 amount of antitoxin will protect a normal animal against 

 a definite amount of toxin. A multiple dose of the 

 serum will protect against multiple doses of the toxin 

 of the same proportion. 



FORMATION OF TOXIN BY BACTERIA. The num- 

 ber of bacteria which are capable of producing a true 

 toxin is quite small. Of those that are concerned in 

 human pathology, diphtheria and tetanus organisms are 

 the most important. For the majority of the other bac- 

 teria toxin formation has not been definitely demon- 

 strated. In most of the other pathogenic bacteria it 

 has been ascertained that the toxic action which they 

 bring about rests upon the poisonous character of the 



