HABITUATION TO TOXINS. 171 



other organisms, in which the only demonstrable 

 change of importance is an increase in the opso- 

 nins, and with this an increase in the power of 

 phagocytic destruction of the cocci. 



In some chronic infections it is possible that the 

 individual shows a resistance to the bacterial 

 toxins, which is on the order of habituation, or 

 adaptation, and which is not represented by 

 any demonstrable antitoxins. Thus, by the use 

 of gradually increasing doses of tuberculin, an 

 individual may eventually tolerate large doses 

 which in the beginning would have been very 

 toxic. 



In spite of this acquired resistance, however, 

 the body appears to form no true antitoxin for the 

 tuberculin. 2 After the cessation of treatment the 

 resistance of the individual gradually returns to 

 normal; that is to say, the cells return to their 

 original susceptibility. 



The possible relation of anaphylaxis to acquired 

 immunity will be discussed in the chapter on 

 "Anaphylaxis." 



Mention may be made here of the well-known 

 but curious phenomenon that resistance may vary 

 with the age of the individual. Typhoid fever at- 

 tacks the adolescent or middle-aged rather 4han 

 the very young or very old. Active tuberculosis 

 grows less common in the later decades of life. 

 Then we have what are distinctively the diseases 

 of childhood : after 15 years of age diphtheria, for 

 example, is uncommon. Some of these instances 



2. Such a course of treatment does cause the formation 

 of antibodies of a specific character, but they appear not to 

 be antitoxic in character. The "antituberculin" which Was- 

 sermann recognizes by means of the method of fixation of 

 complement has not been shown to be an antitoxin. 



