NATURAL IMMUNITY. 497 



dered powerless in the case of any animal, that animal will be 

 immune against the particular organism. This is also borne 

 out by the fact that immunity against a particular organism can 

 be artificially obtained by injections of the toxins of that organ- 

 ism. As a matter of fact, however, natural immunity is in most 

 cases one against infection, i.e. consists in a power possessed by 

 the animal body of destroying the living bacteria when intro- 

 duced into its tissues : such a power may exist though the 

 animal is still susceptible to the separated toxins. We shall 

 now look at these two factors separately. 



I. Variations in Natural Bactericidal Powers. — The fun- 

 damental fact here is that a given bacterium may be rapidly 

 destroyed in one animal, whereas in another it may rapidly 

 multiply and produce morbid effects. The special powers of 

 destroying organisms in natural immunity have been ascribed to 

 (a) phagocytosis, and (1^) the action of the serum. 



(a) The chief factors with regard to phagocytosis have been 

 given above. The bacteria in a naturally immune animal, for 

 example, the anthrax bacillus in the tissues of the white rat, are 

 undoubtedly taken up in large numbers and destroyed by the 

 phagocytes, whereas in a susceptible animal this only occurs to 

 a small extent ; and Metchnikoff has shown that they are taken 

 up in a living condition, and are still virulent when tested in 

 a susceptible animal. The presence or absence of positive 

 chemiotaxis is here also of great importance. The question, 

 however, is whether these differences in chemiotaxis are not 

 themselves capable of explanation. If they are, then the 

 phagocytosis per se is rather the evidence of the presence of 

 immunity than its real essence. An observation of Ehrlich's on 

 hasmolytic sera is somewhat suggestive in this connection. The 

 sera of some animals possess naturally, as above stated, a 

 hsemolytic action on the blood corpuscles of others, and in the 

 cases studied Ehrlich found that this was not due to complement 

 (alexine) alone, but to complement aided by an intermediate 

 body (Zwischenkorper), which behaves in an analogous way to 

 the immune-body of an anti-serum. As already pointed out, 

 bactericidal action closely corresponds with hasmolytic action, 

 and it is quite possible that in a naturally immune animal some 

 intermediate substance may be present which combines with 

 the bacteria and thus produces some change which is evidenced 



