APPLICATIONS OF BACTERIOLOGY 261 



and it has therefore been necessary to suppose that the body is 

 impregnated throughout by some substance produced by the 

 chronic infection. 



The body responds to the attack of the tubercle bacillus by 

 a reaction product, an antibody, and the tuberculin reaction 

 appears to be produced by the coming together or combination 

 (not necessarily in the chemical sense) of this antibody and 

 tuberculin ; that is roughly how the facts are being interpreted 

 nowadays. 



The body by producing this antibody is said to become 

 " sensitive " to tuberculin. The tuberculous individual behaves 

 differently from the healthy subject, being more sensitive, and 

 his excessive susceptibility acting as a warning to him of his 

 danger. It is in this way that the supersensitiveness which is 

 a phase of immunity (v. Chap. XII., page 240) has occasionally 

 been interpreted. There is some resemblance between the 

 tuberculin reaction and the anaphylactic shock ; the diagnosis 

 of tuberculosis by tuberculin is based on the " supersensitive- 

 ness " of the individual. 



The reaction represents in a sense a resistance to the action 

 of tuberculin ; we have seen that the prototype of this reaction, 

 Koch's phenomenon, consists in the expulsion of the re- 

 inoculated virus by the infected subject ; it is therefore also an 

 immunity reaction. 



Tuberculin was at first employed as a remedy, but later only 

 as a diagnostic agent. It was even more studied in veterinary 

 science than in medicine, because it was easier to make experi- 

 ments and control them by autopsies on animals. Even to-day 

 the tuberculin test is quite as important in veterinary as in 

 human practice. 



In the diagnosis of tubercles, nowadays, smaller doses are 

 used for subcutaneous injections than at the time of the first 

 trials of tuberculin. 



The temperature reaches its greatest height about eight 

 hours after injection. A positive reaction indicates merely 

 "tuberculous lesions" and nothing more. It gives no 

 information as to the site or the age of the lesions, nor 



