io6 Immunity 



Metschnikoff found it unnecessary to change his ideas, and 

 persisted in referring all the phenomena to the phagocytes 

 or to enzymes derived from them. 



At this point it will be evident to the reader that the 

 phagocytic theory and the humoral theory contain indubit- 

 able evidence to show that they are important factors in 

 defending the body against invading organisms, and that in 

 each we see mechanisms operative in certain cases. But 

 we have seen that both Metschnikoff and Buchner are 

 obliged to strain a point in order to meet the requirements 

 of increasing knowledge of the subject of immunity. 



Thus, when we come to analyze Buchner 's theory of 

 alexins, we find that if natural immunity depends upon the 

 ability of the alexins to destroy bacteria, that which takes 

 place in vitro should correspond with that which takes place 

 in the animal body, and that the invasion of the animal's 

 body by bacteria should be accompanied by diminution of the 

 bactericidal substance in its blood, which should be used up 

 before the bacteria can be successful in their invasion. Ex- 

 perimental evidence is, however, at hand to show that these 

 requirements may or may not be fulfilled. 



Behring and Nissen* found that there was a definite 

 relation between the bactericidal power of the blood in vitro 

 and the resisting powers of a large number of animals 

 studied, but Lubarschf showed the remarkable exceptions of 

 the rabbit, which is highly susceptible to anthrax, though its 

 blood is highly bactericidal to the anthrax bacillus, and the 

 dog, which is scarcely susceptible to anthrax, though its 

 blood is scarcely bactericidal to the bacillus. 



FliiggeJ found the bactericidal power of the blood greatly 

 lessened in thirty-six hours after anthrax infection, and 

 Nissen that a definite number of bacteria could be killed 

 by a bactericidal serum, after which the alexin became 

 inactive. The diminution of the bactericidal power was 

 shown to occur both in the animal and in the test-tube. He 

 also showed that the reactions of the bactericidal serums 

 were specific, and that when a culture of one kind of bacteria 

 was injected into an animal, the immediate effect was to 

 diminish the activity of the serum for that species, though 

 not necessarily for other species. The diminution of bac- 



* "Zeitschrift fur Hygiene," 1890, vm, S. 412. 

 f "Centralbl. f. Bakt.," etc., 1889, vi, S. 481. 

 % "Zeitschrift fur Hygiene," iv, S. 208. 



