INFECTION AND IMMUNITY. 577 



order) ; while in immunity from such molecular com- 

 plexes as blood-, tissue-, or bacterial cells there are 

 produced receptors of the third order, which act through 

 their haptophore groups as intermediate links between the 

 body to be destroyed and the normally present ferment- 

 like complement that is to bring about the destruction. 

 For all the foreign irritants from which animals have 

 been immunized, be it alien blood, tissue-cells, milk, or 

 bacteria, there is assumed to be circulating normally in 

 the blood a "complement" specifically related to that 

 irritant on the one hand, and to its " receptor " on the 

 other. This idea of plurality for the complement is 

 apparently the vulnerable point in the argument. At 

 all events, it has been vigorously assailed by Bordet and 

 Buchner, especially, who consider the complement as a 

 unit, and who do not regard it as possessed necessarily 

 of specific affinities beyond those common to what may 

 be termed proteolytic enzymes in general ; and Buchner 

 regards it as nothing more than the normally present 

 " alexin," to which he called attention years ago, while 

 with equal warrant Wright might regard it as the 

 "opsonin " on which he has made such instructive studies. 

 Whether these objections be well taken or not, whether 

 the doctrine as a whole can be accepted or not, the ex- 

 perimental data on which it is based justify the opinion 

 that it is the only satisfactory working hypothesis that 

 has been offered in explanation of the mechanism of 

 what Buchner years ago designated the "reactive tis- 

 sue-changes " underlying the establishment of acquired 

 immunity. 1 



The observations serving as the basis for this doctrine 



i Justice cannot be done to tbe beauty and ingenuity 9!" tis con- 

 37 



