THEORIES AS TO ACQUIRED IMMUNITY. 489 



of that organism in the tissues ; this pabulum being ex- 

 hausted, the organisms die out. Such a supposition 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 immunity some- 

 times 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 sub- 

 stance 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 compara- 

 tive anatomist, he saw that it was a very general property 

 possessed by certain cells throughout the animal kingdom 

 that they should take up foreign bodies into their interior 

 and in many cases destroy them. On extending his obser- 

 vations to what occurred in disease, he came to the conclu- 

 sion that the successful resistance of an animal against 

 bacteria depended on the activity of certain cells called 

 phagocytes. In the human subject he distinguished two 

 chief varieties, namely (a) the microphages, which are the 

 " polymorpho-nuclear " leucocytes of the blood, and (^) the 

 macrophages, which include the larger hyaline leucocytes, 



