INFECTION AND IMMUNITY. 547 



cells of the animal organism, the leucocytes, possess 

 the property of taking up, rendering inert, and di- 

 gesting 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 of the tissues on the other. The suc- 

 cess 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. When these means 

 are of sufficient vigor to bring about the death of the 

 bacteria the tissues are victorious ; but when the poisons 

 generated by the bacteria are potent to arrest the pha- 

 gocytic action of the leucocytes then the tissues succumb 

 and infection results. 



Attractive as this doctrine is, plausible as were the 

 arguments in support of it, it is nevertheless, in the 

 light of later evidence, inadvisable to accept it uncondi- 

 tionally in the sense in which it was originally pro- 

 pounded ; 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 in- 

 vestigators indicate that while the leucocytes play an 

 important part in the phenomenon of immunity, it is 

 hardly likely that this always occurs through their 

 taking up and digesting within themselves invading 

 bacteria, as Metchnikoff believed ; but rather that 

 their part in the process is to secrete protective chem- 

 ical substances that are thrown into the circulating 

 blood, and which, in part at least, comprise the de- 



