PASSIVE AGGRESSIVITY 39 



of the infected animal to a tube of serum typically capsulated organ- 

 isms will develop, while the same strain, if grown outside of the body 

 in non-albuminous media, hardly shows any evidence of capsule 

 formation on being transferred to serum. The conclusion hence 

 suggests itself that capsule formation is merely a coincidental evi- 

 dence of a special state which the organism assumes in the animal 

 body and that its increased resistance in the body is not due solely 

 to its capsule, but to the development of a general infectious ability, 

 of which capsule formation and resistance to phagocytosis are asso- 

 ciated, but not necessarily interdependent consequences. Similarly, 

 attenuated organisms of this order are harmless, not merely because 

 they have lost the power to form capsules, but because they are no 

 longer able to assume that special infectious state which may be 

 associated with capsule formation. We could accordingly conceive 

 the existence of a special type of immunity which we may term 

 antiblastic immunity (Ascoli), which would depend upon such an 

 inability of an organism to develop an infectious state, even in 

 the absence of any active antibacterial agencies on the part of the 

 macro organism. 



Organ Virulence. After the virulence of an organism has been 

 artificially raised, one would imagine that this increase would mani- 

 fest itself not only in animals of the same species through which it 

 has been passed, but in others as well. This, however, is not the 

 case, and here as elsewhere in immunological work one meets with 

 remarkable examples of specificity for which no explanation can as 

 yet be given. If, for example, the virulence of the chicken cholera 

 bacillus is increased by passage through the chicken, this increase 

 affects this animal but remains unchanged for the guinea-pig. Simi- 

 larly a certain selective affinity develops for certain organs if the 

 increase in virulence has been brought about through the specific 

 intervention of those organs, and then shows itself irrespective of the 

 manner in which infection is produced. When rats, for example, 

 have been serially infected through the respiratory tract with the 

 lung juice of animals dead with the plague, the virulence of the 

 organisms is not only increased, but plague pneumonia invariably 

 develops on infecting other rats even by the subcutaneous or intra- 

 peritoneal route. 



Virulence of this order which is specifically directed against cer- 

 tain organs is spoken of as organ virulence and is evidently destined 



