" REACTIONS " AND SIMILAR PHENOMENA 305 



index. When an ordinary dose of any vaccine is given to a 

 healthy person the opsonic index undergoes but slight changes, 

 and in particular there is no fall or negative phase. There may 

 be a slight subsequent rise. When the same dose is given to a 

 person infected with the same organism, the negative phase 

 (perhaps preceded by a " false rise ") is most marked, and is 

 followed by a positive rise, or sometimes by a series of rises and 

 falls, gradually dying away like a wave. Similar phenomena can 

 be produced in a healthy person, but here the dose must be much 

 larger. Evidently, therefore, the presence of an infecting agent 

 other than tubercle causes a condition of unstable equilibrium, in 

 which the tissues react in a different manner to healthy ones. 

 And the same condition of altered sensitiveness may persist for 

 long after the disease or injection of a vaccine, so that a dose of 

 dead bacilli that has but little action in health causes a great 

 output of antibodies. This reaction appears to be a general one, 

 occurring with bacteriolysins, agglutinins, etc. 



Attempts have naturally been made to account for a phe- 

 nomenon so remarkable as the tuberculin reaction, and the large 

 number of explanations suggest that none is altogether satisfactory. 

 Many of them do not call for notice. 



Koch's explanation, which was put forward more as a working 

 hypothesis than as an established fact, was this : The bacillus 

 formed a toxin, which diffused outwards from the colonies in the 

 tissues, and when in a sufficient state of concentration set up a 

 coagulation - necrosis going on to caseation. In the zone of 

 tissues just beyond this region the necrosis-producing substance is 

 present, but not in a sufficient degree of concentration to kill the 

 tissues. The injection of a little more of this substance ?.., of 

 tuberculin is sufficient to turn the scale, and a rapid increase of 

 the necrosis takes place. He explains the beneficial effects of 

 the treatment in this wise : The necrotic tissue does not form a 

 suitable medium of growth for the tubercle bacillus (which is but 

 rarely seen in caseous material), and the extension of the process 

 may lead to the complete enclosure of the bacteria in dead and 

 altered tissues, in which they are incapable of further growth. 



This theory assumes that the substance which produces necrosis 

 is identical with the active principle of tuberculin ; but tuberculin 

 in large doses will not produce necrosis in a healthy animal. It 

 seems also to fail to account for the remarkable rise in the 

 temperature, since it occurs in patients who are not febrile, as 



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