164 BACTERIA IX RELATION TO DISEASE 



by heat (50- 70) or by the addition of chemical substances such as 

 trichloride of iodine or carbolic acid. Finally, inoculation with the at- 

 tenuated bacteria themselves may be resorted to. All the methods lead to 

 the same results, and the principle involved in each case is the same, 

 for the treatment of bacteria with substances that hinder their vegetative 

 development naturally reduces also the amount of poison they produce. 

 It was the method of immunizing by means of weakened pure cultures that 

 formed the starting-point of the treatment with the toxines themselves. 

 Behring (150), by using tetanus bacilli that were attenuated in different 

 degrees by iodine trichloride, was able in about seventy-five days to 

 immunize a horse to such an extent that it endured the injection of looc.c. 

 fully virulent tetanus culture. An untreated animal was killed by 0-5 c.c. 

 of the same culture. To attain this degree of immunity several hundred c.c. 

 of attenuated cultures were necessary. It took to render a horse highly 

 immune to diphtheria eighty days and eight hundred c.c. of filtered virulent 

 diphtheria toxine (151). It could then be tapped for curative serum. By 

 continued treatment the resistance to virus can be still further increased and 

 persists for a considerable time. In tetanus-immune horses the immunity 

 lasts two years, and in diphtheria-immune horses about the same time. In 

 experiments with other bacterial poisons the immunity attained was not 

 so persistent, although enduring many weeks or months. Variations are 

 unavoidable. 



Assuming that artificial immunity is only the acquired toleration to the 

 poisons, its disappearance would be explained by the poison gradually leaving 

 the body and finally disappearing entirely. The cells of the body would 

 thus gradually lose their power of toleration. The fact that the immunity 

 lasts two years and more is no argument against this, for we know that 

 some poisons are excreted from the body with great slowness ; mercury, for 

 instance, does not disappear for six months or more after administration. 

 Immunization by injection of toxines must be regarded as an experimental 

 chronic intoxication. 



Such an artificially-induced immunity protects of course not only from 

 the direct administration of the poison, but also from the virus excreted by 

 invading bacteria. Tetanus bacteria, for instance, are not arrested in their 

 growth by the antitoxine, but their poisons are neutralized. The same 

 takes place in diphtheria (152), while in other diseases it is not yet clear 

 whether the neutralization of the poison is not accompanied by arrest of 

 growth of the bacteria (Cholera, p. 167). 



The immunity produced is strictly specific ; the diphtheria poison, for 

 instance, imparts immunity only against diphtheria, not against tetanus or 

 anthrax, or any other infections. This again can be easily explained by 

 toleration, which is in fact competent to account for all the phenomena 

 which the immunized animal exhibits in its own organism. Towards other 



