SUSCEPTIBILITY AND IMMUNITY. 269 
temperature the antitoxin is destroyed, subsequent inoculations 
showed that the toxin was still active. 
The experiments of Stern (1894) show that the typhoid bacil- 
lus not only grows in blood-serum from a typhoid convalescent, which 
has been proved to neutralize its pathogenic effects when injected 
into a susceptible animal, but also that its toxic products are de- 
veloped in this culture medium. From this Stern concludes that the 
serum must in some way act upon the infected animal, causing 
changes which enable it to resist infection, rather than upon the 
bacillus or upon its toxic products directly. It has also been shown 
by Behring (1890) for the diphtheria bacillus, by Vaillard for the 
tetanus bacillus (1892), and by Issaeff (1893) for the micrococcus of 
pneumonia, that these several pathogenic microdrganisms may be 
cultivated in the blood-serum of animals immunized for the diseases 
which they produce. 
In a paper published in 1897, Ehrlich advanced his “ side-chain ” 
(seitenkette) theory. He considers the individual cells of the body to 
be analogous, in a certain sense, to complex organic substances, and 
that they consist essentially of a central nucleus to which secondary 
atom-groups having distinct physiological functions are attached by 
“side chains ”—such as chemists represent in their attempts to illus- 
trate the reactions which occur in the building up or pulling down of 
complex organic compounds. The cell-equilibrium is supposed to be 
disturbed by injury to any of its physiological atom-groups—as by a 
toxin—and this disturbance results in an effort at compensatory repair 
during which plastic material in excess of the amount required is 
generated and finds its way into the blood. This Ehrlich regards as 
the antitoxin, which is capable of neutralizing the particular toxin 
to which it owes its origin, if this is subsequently introduced into the 
blood. In this theory a specific combining relation is assumed to 
exist between various toxic substances and the secondary atom- 
groups of certain cellular elements of the body. The atom-groups 
which, in accordance with this theory, combine with the toxin of any 
particular disease germ, Ehrlich calls the “toxophoric side chain.” 
Immunity, according to Ehrlich, is either “active” or “passive.” 
Passive immunity resalts from the introduction of the immunizing 
substance from an immunized animal into the circulation of a non- 
immune animal, e.g., the use of diphtheria antitoxin as a prophy- 
lactic. This passive immunity is more transient than the active 
immunity which results from an attack of an infectious disease, from 
inoculations with living vaccines, or from repeated injections of in- 
creasing doses of the toxins of pathogenic bacteria. Hhrlich’s ex- 
