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.