Acquired immunity against micro-organisms 291 



these animals refractory to infection by the diphtheria bacillus though 

 they became more susceptible to intoxication. Von Behring considers 

 that this augmentation of susceptibility to the diphtheria poison may 

 be a means of rendering the local reaction of the living elements at 

 the point of introduction of the bacilli more active. 



In any case, it is beyond question that acquired immunity against 

 microbial infection is quite independent of the resistance against the 

 toxins of the corresponding micro-organism. An antitoxic manifesta- 

 tion of any kind, therefore, cannot be regarded as necessary for the 

 development of immunity against the micro-organism. 



Of all the humoral properties developed in acquired immunity 

 against micro-organisms, the fixative power and the protective power 

 are the most constant. It might naturally be suggested, as a result 

 of this observation, that these two powers are indispensable for the 

 manifestation of phagocytosis for the purpose of destroying and of 

 ridding the animal of the pathogenic organisms. It is quite possible 

 to understand how, under these conditions, the idea has been put 

 forward that anti-infective acquired immunity is the result of two 

 different factors : in the first place, a humoral property independent 

 of the phagocytes and, in the second place, the phagocytes themselves. 

 But the part played by these cells cannot be accepted as purely 

 secondary a view which has been advanced and defended again and 

 again. This question is of such importance that it is reasonable to 

 ask whence come the humoral properties, such as the fixative power 

 and the protective power, factors of such far-reaching influence in 

 anti-infective immunity ? 



Thanks to the work of several investigators this question may now [306] 

 be answered. Pfeiffer and Marx 1 first supplied important informa- 

 tion concerning the origin of the protective property. Into rabbits 

 they made subcutaneous inoculations of cholera vibrios, killed by 

 heat (70 C.), and then examined, most minutely, the protective power 

 of the blood and of extracts from various organs. Examining, sepa- 

 rately, the protective power of the serum and that of the layer of 

 leucocytes deposited in tubes, Pfeiffer and Marx were unable to find 

 any marked difference. Nor did they ever obtain any definite effect 

 with leucocytes collected from pleuritic exudations. From these 

 observations they concluded that the leucocytes of the blood could 

 not be regarded as the source of the protective substance (or 



1 Ztschr f. Hyg., Leipzig, 1898, Bd. xxvn, S. 272. 



192 



