DEFENSIVE FACTORS OF THE ANIMAL ORGANISM 243 



In other cases 5 as in. the instance of the malaria-immunity of 

 negroes the resistance seems to be acquired rather than inherited, 

 for, as Hirsch was first to note, death from this disease occurred 

 frequently among the children, while adult negroes were rarely 

 attacked. 



DIFFERENCES IN INDIVIDUAL RESISTANCE. In bacteriological ex- 

 perimentation with smaller test animals, a direct ratio may often 

 exist between body weight and dosage in determining the outcome of 

 an infection, provided the mode of inoculation has been the same 

 and the virulence of the germ not excessive. It would seem, therefore, 

 that among these animals the difference in resistance in the face of 

 an artificial infection between individuals of the same race is very 

 slight. 



In higher animals, however, especially in the case of man, the 

 existence of such apparent individual differences is a well-established 

 fact, although in judging of them we must not forget that the condi- 

 tions of infection are not subject to the uniformity and control which 

 animal experimentation permits. Of a number of persons exposed to 

 any given infection there are always some who are entirely unaffected 

 and there are great variations in the severity of the disease in those 

 who are attacked. In the absence of positive evidence in support of 

 the direct inheritance of this individual immunity, the most reasonable 

 explanation for such differences in resistance seems to lie in attributing 

 them to individual variations in metabolism or body chemistry. De- 

 pressions, for instance, in the acidity of the gastric secretion would 

 predispose to certain infections of gastro-intestinal origin. Anatomical 

 differences, too, may possibly influence resistance. Thus, Birch-Hirsch- 

 feld believed that certain anomalous arrangements of the bronchial 

 tubes predisposed to tuberculosis. 



Instances of transient susceptibility induced by physical or mental 

 overwork, starvation, etc., should hardly be classified under this head- 

 ing, since the conditions in such cases correspond simply to experi- 

 mental depression of natural species for race resistance. 



Acquired Immunity. It is a matter of common experience that 

 many of the infectious diseases occur but once in the same individual. 

 This is notably the case with typhoid fever, yellow fever, and most 

 of the exanthemata, and is too general an observation to require exten- 

 sive illustration. A single attack of any of the diseases of this class 



6 Halin, in Kolle und Wassermann, loc. cit. 



