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.