ESMOND R. LONG 1033 



destroys the activity of the substance, although possibly slight changes in the mole- 

 cule can be borne without loss of potency. 



The exact mechanism of the tuberculin reaction is not understood. There is some 

 evidence that tuberculous tissue alters tuberculin in such a way as to render it toxic, 

 while normal tissue has no such effect. Thus it is toxic only to the tuberculous host. 

 Careful experiments on highly sensitized, non-tuberculous tissues of the tuberculous 

 animal, viz., the skin and testis, have failed to show that these sensitive tissues have 













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Fig. 7. — Glomerular hemorrhage and exudation in kidney of a tuberculous hog four days after 

 renal perfusion with a suspension of tuberculin protein. 



any power whatsoever to alter or neutralize tuberculin. We can only say at present 

 that tuberculin is a specific irritant for these cells. 



The tuberculin reaction is of high but not absolute specificity. Anatomically it is 

 characterized by all the features of acute inflammation, in which increased capillary 

 permeability appears to play an important part. The specificity of the reaction is 

 sufficiently high to make the tuberculin reaction reliable in the recognition of tuber- 

 culous infection in cattle. The widespread incidence of latent tuberculous infection 

 in adult man, conferring sensitiveness to tuberculin, goes far to nullify the value of 

 tuberculin in the recognition of active tuberculosis, although quantitative tuberculin 

 reactions may be suggestive. Tuberculin is useful in the diagnosis of active tuber- 

 culosis in children, and invaluable in epidemiological studies of the incidence of in- 

 fection in children. 



