THEORIES AS TO ACQUIRED IMMUNITY. 449 



They also show the futility of attempting, even now, to 

 make a general statement which would be applicable to all 

 cases. One or two of these theories may, however, be 

 mentioned, as they are of interest in connection with the 

 development of this subject. 



1. The Theory of Exhaustion, with which Pasteur's name 

 is associated, supposes that in the body of the living 

 animal there are substances necessary for the existence of 

 a particular organism, which become used up during the 

 sojourn of that organism in the tissues; this pabulum 

 being exhausted, the organisms die out. Such a supposi- 

 tion is, of course, quite disproved by the fact of passive 

 immunity, as a small quantity of serum in which the 

 pabulum has been exhausted cannot lead to its exhaustion 

 in the serum of another animal. 



2. The Theory of Retention supposes that the organisms 

 within the body produce substances which are inimical to 

 their growth, so that they die out, just as they do in a 

 test-tube culture before the medium is really exhausted. 

 In its simple form, the theory is scarcely tenable, as it 

 would be difficult to conceive how such substances could 

 be retained in the body for so many years as acquired im- 

 munity sometimes lasts. In a modified form, however, it 

 might include theories still held and which are founded 

 on the facts of passive immunity. It might, for example, 

 include the theory of Buchner, according to which the 

 antitoxic substance in the serum is merely a modified 

 toxine, which has the power of producing in another 

 animal a rapid reaction resulting in immunity. The facts 

 stated above with regard to the production of antitoxine 

 are, however, quite opposed to such a supposition. 



3. The Theory of Phagocytosis. This is the theory 

 which was brought forward by Metchnikoff to explain 

 the facts of natural and acquired immunity, and which has 

 been of enormous influence in stimulating research on this 

 subject. Looking at the subject from the standpoint of the 

 comparative anatomist, he saw that it was a very general 

 property possessed by certain cells throughout the animal 



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