128 IMMUNITY AND INFECTION 
adsorption quite unlike the reaction of a weak acid and a weak base. 
Michaelis/ however, believes that the evidence of colloidal adsorption 
is insufficient at the present time to warrant this supposition. 
The hiuiioral theory of immunity fails to attribute any prominent 
part in immunity to phagocytic cells other than the removal of dead 
bacteria. No theory had been advanced, up to the present time, 
which explains all the phenomena of humoral immunity, and it is quite 
clear that the Ehrlich theory is at best but an attempt to explain pro- 
found chemical and physico-chemical phenomena by inadequate 
symbolism. Yet it must be remembered that this side chain theory has 
stimulated a vast amount of work and in spite of all this work no 
acceptable substitute has thus far been advanced to take its place. 
Lacking such a substitute it has seemed proper to retain the side chain 
theory practically in its entirety. 
•• 4W q^*' «J 
>*: ** 
# 
Ba 
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*^ 
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M. 
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Fig. 8. — Phagocytosis of gonococcus. 
B. The Cellular or Phagocytic Theory of Immunity.-— The cellular 
theory of immunity, formulated and championed !)y Metchnikoff, 
had its inception in observations of the nutritive activities of amoebse, 
which could be watched under the microscope. It was observed that 
these simple, transparent protozoa, when about to feed, approached 
and flowed around a minute organism, as a bacterial cell. Shortly 
after engulfment the contour of the ingested bacterium lying within 
the substance of the amoeba became less and less distinct and finally 
1 Die Bindungsgesetze von Toxin und Antitoxin, Berlin, 19D5. 
2 See Metchnikoff (Immunity in Infectious Diseases, translated by Binnie, New York) 
for summary. 
