626 ANAPHYLAXIS IN RELATION TO INFECTION AND IMMUNITY 



from 4 per cent, among infants under three months of age to 70 per cent, 

 among children from eleven to fourteen years. This explains, in part 

 at least, the relatively high resistance of children to tuberculin, the 

 difficulty there is said to be in eliciting reactions, and the necessity that 

 exists for using large doses. Usually, when children fail to react, it is 

 because they are not tuberculous or because the lesion is too small, 

 whereas in later years, until adult life is reached, the reaction is observed 

 with increasing frequency and with smaller doses, because the incidence 

 of infection increases progressively from 5 per cent, during infancy to 

 90 per cent, and over in adult life. 



The prevalence of tuberculosis, however, by no means indicates that 

 the infected individual suffers ill health or will succumb to the infection. 

 An individual may be enjoying excellent health, and still harbor a tu- 

 berculous lesion, and display a marked degree of hypersensitiveness to 

 tuberculin. Such a person is not usually regarded as tuberculous until 

 there are tangible symptoms referable to its existence. It is important 

 to remember that tuberculin is an index of tuberculous infection, and not of 

 disease in a clinical sense. Numbers of persons and cattle reacting to 

 tuberculin remain healthy and do not develop symptoms of disease, the 

 autopsy disclosing the presence of inactive or regressing lesions. 



In former years it was considered possible to obtain false positive 

 reactions in convalescents and patients in an enfeebled condition who 

 were non-tuberculous, and also in other diseases, such as syphilis, 

 leprosy, and actinomycosis. More accurate anatomic statistics and 

 careful studies of the tuberculin test administered to a large number of 

 individuals, healthy, tuberculous, and sufferers from other diseases, 

 have gradually changed the attitude of the profession and served to 

 establish the high specificity of the tuberculin reaction. 



Sources of Error in the Tuberculin Reaction. From the foregoing 

 it will readily be understood that most errors in the tuberculin reaction 

 refer to false negative rather than to false positive reactions. 



False positive reactions may be observed in leprosy, where the bacillus 

 bears such close morphologic and biologic resemblance to the tubercle 

 bacillus, and it is likewise true that massive doses of tuberculin injected 

 subcutaneously may produce a toxic fever in debilitated individuals, 

 but positive reactions in healthy individuals can usually be ascribed 

 (a) to a small hidden tuberculous lesion or (b) to a healed tuberculous 

 lesion. As just stated, tuberculin simply indicates hypersensitiveness to 

 the tubercle protein, and this may exist with a very small unimportant 

 lesion, or persist after a lesion has been " healed" to the extent of en- 

 capsulation. 



