INFECTION AND IMMUNITY. 583 



cells of the animal organism, more especially the leuco- 

 cytes, possess the property of taking up, rendering 

 inert, and digesting micro-organisms which they may 

 encounter in the tissues. Metchnikoff believed that in 

 this way immunity from infection may in many, if not 

 all, cases be explained. He believed that suscepti- 

 bility to, or immunity from, infection was essentially a 

 matter between the invading bacteria on the one hand, 

 and the leucocytes and the tissue cells on the other. 

 The success or failure of the leucocytes in protecting 

 the animal against infection depends, according to this 

 doctrine, entirely upon the efficiency of the provisions 

 possessed by them for destroying bacteria, and upon the 

 aggressive powers of the invading organisms. When 

 the activity of the body cells is of sufficient vigor to 

 bring about the death of the bacteria the tissues are 

 victorious ; but when the poisons generated by the bac- 

 teria are potent to arrest the phagocy tic action of the leu- 

 cocytes then the tissues succumb and infection results. 



THE ALEXIN THEORY OF BUCHNER. Attractive as 

 this doctrine is, plausible as were the arguments in sup- 

 port of it, it is nevertheless, in the light of later evi- 

 dence, inadvisable to accept it unconditionally in the 

 sense in which it was originally propounded ; in fact, 

 Metchnikoff himself has in recent years seen fit to adopt 

 certain modifications of his views as first expressed. 

 The later studies of a number of investigators indicate 

 that while the leucocytes play an important part in the 

 phenomenon of immunity, especially in natural immu- 

 nity, it is hardly likely that this always occurs through 

 their taking up and digesting within themselves inva- 

 ding bacteria, as Metchnikoff believed ; but rather that 

 their part in the process is to secrete protective chemical 



