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 microorganisms 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 results 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. Ehrlich's ex- 



