IMMUNITY. 167 



tively insusceptible to tuberculin, but is easily invaded by the tuber- 

 cle bacillus, and after becoming infected with the germ he loses his 

 insusceptibility to the toxin. It may be seen from the illustrations 

 that have already been given that inability on the part of the micro- 

 organisms to grow in the animal body is an important factor in nat- 

 ural immunity, but this is by no means the sole factor in the estab- 

 lishment of this condition. 



In the chapter on the germicidal constituents of the blood we 

 have shown the existence of alexins not only in this fluid but in 

 other tissues and juices of the body. It was at one time supposed 

 that the alexins of the blood played a large part in the establish- 

 ment of natural immunity, but more extended investigation has 

 shown that this supposition is erroneous. The blood serum of the 

 rabbit has relatively strong bactericidal effect upon anthrax baxjilli, 

 and yet this animal is most highly susceptible to this infection. 

 Moreover, Lubarsch ' has shown that the bactericidal action of rab- 

 bit's blood on the anthrax bacillus is more marked outside than it is 

 inside the body. The discovery of the germicidal effect of rat's 

 blood on anthrax bacilli was at one time suggested by Behring,* as 

 an explanation of the relative immunity possessed by this animal to 

 anthrax infection. However, more extended observation showed 

 that the white rat is more susceptible to anthrax than was at one 

 time supposed, and yet the bactericidal action of rat's blood on the 

 anthrax bacillus can be demonstrated not only outside the animal 

 body, but also within the organism. If anthrax bacilli be mixed 

 with rat's blood and the mixture be injected into mice, the latter 

 animals escape infection, but the rat from which the bactericidal 

 blood was obtained succumbs to infection with the bacillus. On the 

 other hand, the adult dog is immune to anthrax infection, while the 

 blood serum of this animal has no germicidal action on this organ- 

 ism. Indeed, anthrax bacilli grow abundantly in the blood serum 

 of the dog. It will thus be seen that the blood of the rabbit, an 

 animal highly susceptible to anthrax infection, has marked germicidal 

 action on the anthrax bacilli outside the body ; while the blood of 

 the dog, an animal immune to anthrax infection, furnishes a good 

 culture medium for the growth of the bacillus. We must, therefore, 

 conclude that the attempt to explain natural immunity solely by the 

 bactericidal properties of the blood must be abandoned. Indeed, 

 it is probable that the germicidal properties of the blood do not con- 

 stitute a very important factor in the production of natural immunity. 

 However, we do not claim that the alexins can be whoUy disre- 

 garded in the study of this problem, and after bringing forward 

 another theory concerning natural immunity, we will return to this 

 subject. 



' GentrdlMatt f. Bahteriologie, 6, 1889. 

 ^ Zeitschrift Jur Hygiene, 9, 1890. 



