IMMUNITY AND SUSCEPTIBILITY. 85 



were 300 immunizing units of diphtheria antitoxin, die 

 of typical symptoms of diphtheria-poisoning after the ad- 

 ministration of a comparatively small dose of the toxin. 

 From all that has gone before it must be clear to the 

 reader that no single theory thus far advanced can ex- 

 plain immunity. Acquired immunity may depend in 

 the great majority of cases upon antitoxins, but as yet 

 we have no satisfactory explanation of natural immunity. 

 The humoral theory may be applicable in some cases ; in 

 others one cannot deny the importance of the role played 

 by the phagocytes. 



