Defensive Proteins, etc. 109 



theory and the humoral theory contain indubitable evidence that 

 both the body cells and humors are important factors in defending 

 the body against invading organisms, and that in each we see mechan- 

 isms 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 vivo, and that the invasion 

 of the animal's body by bacteria should be accompanied by diminu- 

 tion of the bactericidal substance in its blood, which should be used 

 up before the bacteria can be successful in their invasion. Experi- 

 mental evidence is, however, at hand to show that this is not always 

 true. 



Behring and Nissen* found that there was a definite relation be- 

 tween 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. 



Flugget 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 bac- 

 tericidal 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 bactericidal energy was shown 

 by him to depend upon the presence of the bacteria, as the injection 

 of filtrates of bacterial cultures did not affect the bactericidal 

 properties of the serum. This was a very important observation. 



There is a correspondence between the behavior of the phagocytes 

 and the body juices. When the activity of the phagocytes toward 

 the bacteria is increased, the bactericidal activity of the serum is 

 usually intensified. But immunity is only partly explained by 

 alexins and bacteriolysis, for it embraces the ability of the organ- 

 ism to endure the effects of toxins some of which are in no way 

 connected with bacteria. 



Tolerance to certain toxins is, of course, natural to many animals, 



* "Zeitschrift fur Hygiene," 1890, vra, 412. 

 t "Centralbl. f. Bakt.," etc., 1889, VI, 481. 

 { "Zeitschrift fiir Hygiene," iv,2o8. 



