SUSCEPTIBILITY AND IMMUNITY. 237 



THEORIES OF IMMUNITY. 



Exhaustion Theory. For a time Pasteur supported the view 

 that during an attack of an infectious disease the pathogenic micro- 

 organism, in its multiplication in the body of a susceptible animal, 

 exhausts the supply of some substance necessary for its development, 

 that this substance is not subsequently reproduced, and that conse- 

 quently the same pathogenic germ cannot again multiply in the body 

 of the protected animal. This view is sustained in a memoir pub- 

 lished in the Comptes Bendus of the French Academy in 1880, in 

 which Pasteur says : 



" It is the life of a parasite in the anterior of the body which produces the 

 malady commonly called ' cholera des ponies, 1 and which causes death. 

 From the moment when this culture (i.e., the multiplication of the parasite) 

 is no longer possible in the fowl the sickness cannot appear. The fowls are 

 then in the constitutional state of fowls not subject to be attacked by the 

 disease. These last are as if vaccinated from birth for this malady, because 

 the foetal evolution has not introduced into their bodies the material neces- 

 sary to support the life of the microbe, or these nutritive materials have 

 disappeared at an early age. 



"Certainly one should not be surprised that there may be constitutions 

 sometimes susceptible and sometimes rebellious to inoculation that is to 

 say, to the cultivation of a certain virus when, as I have announced in my 

 first note, one sees a preparation of beer yeast made, exactly like one from 

 the muscles of fowls (bouillon), to show itself absolutely unsuited for the cul- 

 tivation of the parasite of fowl cholera, while it is admirably adapted to the 

 cultivation of a multitude of microscopic species, notably to the bacteride 

 charbonneuse (Bacillus aiithracis). 



' ' The explanation to which these facts conduct us, as well of the consti- 

 tutional resistance of some individuals as of the immunity produced by 

 protective inoculations, is only natural when we consider that every culture, 

 in general, modifies the medium in which it is effected a modification of 

 the soil when it relates to ordinary plants; a modification of plants and ani- 

 mals when it relates to their parasites ; a modification of our culture liquids 

 when it relates to mucedines, vibrionie'/is, or ferments. 



" These modifications are manifested and characterized by the circum- 

 stance that new cultivations of the same species in these media become 

 promptly difficult or impossible. If we sow chicken bouillon with the mi- 

 crobe of fowl cholera, and, after three or four days, filter the liquid in order 

 to remove all trace of the microbe, and subsequently sow anew in the fil- 

 tered liquid this pai'asite, it will be found quite powerless to resume the most 

 feeble development. The liquid, which is perfectly limpid after being fil- 

 tered, retains its limpidity indefinitely. 



"How can we fail to believe that by cultivation in the fowl of the atten- 

 uated virus we place its body in the state of this filtered liquid which can 

 no longer cultivate the microbe ? The comparison can be pushed still 

 further; for if we filter the bouillon containing the microbe in full develop- 

 ment, not on the fourth day of culture, but on the second, the filtered liquid 

 will still be able to support the development of the microbe, although with 

 less energy than at the outset. We comprehend, then, that after a cultiva- 

 tion of the modified (attenue) microbe in the body of the fowl we may not 

 have removed from all parts of its body the aliment of the microbe. That 

 which remains will permit, then, a new culture, but in a more restricted 

 measure. 



' ' This is the effect of a first inoculation ; subsequent inoculations will 



