THEORIES OF IMMUNITY. 



the leucoeytic secretion of the alexines, but 

 he went a step further than had before been the 

 case; he admitted that phagocytic activity has a 

 decisive importance in many cases in overcom- 

 ing the infectious processes, particularly in those 

 in which the alexines secreted have been unable 

 to produce more than a passing diminution in 

 the vitality of the bacteria. In such conditions 

 the bacteria have been so modified that their 

 chemical functions have been transformed into 

 a latent condition from which they would be 

 able to revive in full vital energy if the phago- 

 cytes were not present to prevent them from 

 doing so. In any case, as Metchnikoff says, this 

 conception is far from Buchner's original theory, 

 according to which the phagocytes are to be 

 considered only as capable of englobing dead or 

 harmless bacteria. He further speaks of other 

 marked changes in opinion which have occurred, 

 and sums up the present condition of things as 

 follows: the theory of phagocytosis has been 

 strengthened by the demonstration of the follow- 

 ing facts, that the phagocytes, in cases of immu- 

 nity, engj.obe and destroy living and virulent 

 bacteria, without these having been previously 



36 



