THE DEFENSES OF THE BODY 305 



is apparently the vulnerable point in the argument 

 (see above ''On the Specificity of Complement")- At all 

 events, it has been vigorously assailed by Bordet and 

 Buchner, especially, who as said above, consider the 

 complement a unit, and who do not regard it as pos- 

 sessed necessarily of specific affinities beyond those com- 

 mon 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 

 experimental 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 tissue-changes" 

 underlying the establisment of acquired immunity. 1 Ehr- 

 lich's conception may be graphically represented as follows: 



The observations serving as the basis for this doctrine 

 have given to the blood and fluids of the body a new and 

 peculiar interest. According to circumstances, there may 

 be detected in the blood and tissue juices a number of mole- 

 cular complexes having totally different functions and 

 affinities, and therefore presumably different from one 

 another: 



First, there is normally present in the blood serum of 

 practically all animals the defensive "alexins" already 

 mentioned. 



1 Justice cannot be done to the beauty and ingenuity of this conception 

 in so brief a summary as is appropriate to a text-book. To be appreciated it 

 must be read as it came from the authors. 

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