464 BACTERIOLOGY. 



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 

 success or failure of the leucocytes in protecting the 

 animal against infection depends, according to this doc- 

 trine, entirely upon the efficiency of the means 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. 



Has this doctrine of phagocytosis, as advanced by 

 Metchnikoff, stood the test of experimental criticism? 

 Evidence that has accrued since the time of its sugges- 

 tion has rendered questionable the advisability of its 

 unconditional adoption in the strict sense that Metch- 

 nikoff propounded it. The later studies of a number 

 of investigators indicate that while the leucocytes play 

 a most important part in the phenomenon of immunity, 

 it is hardly likely that this always occurs through their 

 taking up within themselves and digesting invading 

 bacteria, as Metchnikoff believes, but rather that their 

 part in the process is to secrete protective chemical sub- 

 stances that are thrown into the circulating blood, and 

 which, in part at least, comprise the defensive bodies to 

 which Buchner has given the name " alexines." 



The first severe blow that Metchnikoff' s theory of 

 phagocytosis received was given by Nut-tall, 2 in his 



1 See Hahn. Arch, fur Hygiene, 1895, Bd. xxv, p. 105. 



2 Zeitschrift fur Hygiene, 1888, vol. iv. 



