BACTERICIDAL OR ANTI-BACTERIAL IMMUNITY. 189 



disease producing, and against which, therefore, the body is not, in the old sense, 

 immunized. 



But these new uses of the word are, I think, unfortunate because the word immu 

 nity lias come to have a special and useful significance in relation to infection, intoxica- 

 tion, and other conditions of natural or acquired tolerance to obviously and seriously 

 harmful agents. The process in both instances is, indeed, one of adaptation, and the 

 newly acquired capacities of the serum are due to substances resulting from this adap- 

 tation. They arise from a functional modification of parts of the body, and hence may 

 be appropriately called adaptive substances. It seems to the writer that it would be 

 better to consider immunization as a special phase of adaptation, and so limit the appli- 

 cation of the word that it shall still connote infection and intoxication in the traditional 

 sense. 



This, then, is the rationale in accordance with Ehrlich's hypothesis of the action of 

 these cytolytic or cytotoxic substances, either existing naturally, as they do in some 

 animals, or being called forth in larger quantities in the process of adaptation to the 

 cells which they destroy. This view lias been most fully tested upon haemolytic sera, 

 since here the reaction is most easily studied. But so far as one can see, it applies as 

 well to the phenomena of baceriolysis, whose direct study is much more difficult. It 

 should, however, be borne in mind that the erythrocytes are very delicate and very 

 peculiarly constituted cells, and it is possible that inferences drawn from haemolysis are 

 not applicable without qualification to other and less vulnerable cell types. 



The origin of the amboceptors of these cytotoxic substances is accounted for in the 

 same way as in the case of antitoxin. The cells or substances which are introduced 

 into the animal, and to which it proceeds to adapt itself, lead, through union with such 

 body cell receptors as may be fitted to them, to the overproduction of these special com- 

 plex receptors. These are presently cast off as superfluous to the body cell producing 

 them, and are then free as amboceptors in the body fluids. 



As in the case of antitoxin formation, it is probable that the cell receptors which 

 are thus increased are normally concerned in cell assimilation, and it is not unlikely that 

 their complex character may have some relationship with the complexities of the 

 "giant" albuminous molecules, which must suffer initial changes before becoming fit 

 for assimilation. At any rate this hypothesis assumes that in the process of adaptation 

 either to toxic substances or to foreign cells or other albuminous material, the body 

 develops no new capacities, but only an exaggeration of those already existing. 



As to the exact source of the amboceptors in artificially immunized animals we can 

 not yet speak with certainty. 



The Action of Phagocytes in Cytolysis. It was inevitable that the remark- 

 able studies just summarized on cytolytic sera should have led to a clearer conception 

 of the manner in which phagocytes destroy bacteria and other organic substances. It 

 is no longer permissible to hold as distinct and unrelated processes the action of phago- 

 cytes and the action of the body fluids in the destruction of foreign substances in the body. 



Metschnikoff, the learned and able advocate of the importance of phagocytosis in 

 the protection of the body against micro-organisms, now recognizes the importance of 

 the adaptive substances, some of which may be largely increased in amount in the proc- 

 esses of immunization More strenuously than other observers, however, he insists 

 upon the phagocytic cells, especially the leucocytes, as the originators of the substances 

 concerned in cytolysis, arid holds that under ordinary conditions it is only within these 

 cells that these substances are effective. In artificially immunized animals, however, 

 the intermediary substances, it is conceded by Metschuikoff , may be set free from the 

 cells which produce them and mingle with the body fluids. The complement, on the 

 other hand, which he, in common with others of the French school, calls cytase, in recog- 

 nition of its ferment-like characters, Metschnikoff does not believe to be set free in the 

 body fluids except through some damage to the leucocytes in which it is formed. Such 

 a damage, for example, as befalls the leucocytes in the clotting of the blood : for in this 

 process it is assumed that the setting free of the fibrin ferment involves the destruction 

 phagolysis of the leucocytes. 



