140 THE DIFFERENT TYPES OF IMMUNITY 



in a single one of the naturally immune animals, it is not unlikely 

 that the same mechanism may be operative in the production of 

 natural immunity in others, and especially in connection with those 

 organisms which, like the anthrax bacillus, are characterized by a 

 high degree of infectiousness and a low grade of toxicity. In the 

 case of some of these, it probably also plays a role in the develop- 

 ment of an acquired immunity. 



Antibacterial Immunity. In the majority of infections with the 

 semiparasites, on the other hand, the acquired immunity is not 

 antiaggressive, but bactericidal in character, and since bactericidal 

 influences may be exercised either by the serum alone or the leuko- 

 cytes alone, or by both in combination, the resultant immunity 

 may, theoretically at least, be due to an exaggerated functional 

 activity of either one or both of these factors. It is not my purpose 

 at this place to enter into a discussion of the question which one of 

 the two is really the primum movens in the production or existence 

 of antibacterial immunity. I would merely recall that for many 

 years immunity students were divided into two opposing factions, 

 viz., the humoral school, led by Pfeiffer, and the older phagocytic 

 school, represented by Metschnikoff, whose respective standpoints 

 seemed for a long time irreconcilable the one with the other. At 

 the present time the original sharp lines between the two schools 

 have fallen, and we recognize that there is an intimate interrelation- 

 ship between the cellular and the humoral defences, that the two 

 supplement one another and that neither alone should be viewed 

 as sufficient to protect an animal against infection and its con- 

 sequences. But while recognizing the importance of both, w r e must 

 also admit that neither the one nor the other seems to be solely 

 responsible for the development of an acquired immunity. 



There can be no doubt that as a result of infection or vaccination, 

 corresponding bacteriolytic amboceptors are formed in large quantity 

 and that the serum of such animals in the test-tube is capable of 

 destroying the corresponding organisms in large numbers, and that 

 the same can occur in the living animal, but we must also recognize 

 the fact that this flood of bacteriolysins does not remain while the 

 increased resistance which has been established may last for years. 

 That the phagocytic influences in such cases do not play a more 

 important part than the bacteriolysins, can readily be shown by 

 studying the opsonic curve, which never remains above the normal 



