IMMUNITY AGAINST TETANUS 385 



in the work of Behring and Kitasato in Germany, and of Tizzoni 

 and Cattani in Italy. The former observers found that such an 

 immunity could be conferred by the injection of very small and 

 progressively increasing doses of the tetanus toxin. The degree 

 of immunity attained, however, was not high. Subsequent 

 work has shown that the less rich a crude toxin is in modifica- 

 tions of the true toxin the less useful it is for immunisation 

 procedures. In fact it is doubtful if small animals can be 

 immunised at all by fresh filtrates. In some cases it has been 

 found that the injection of non-lethal doses instead of commenc- 

 ing an immunity actually increases the susceptibility of the 

 animal. More successful are the methods of accompanying the 

 early injections of crude toxin with the subcutaneous introduction 

 of small doses of iodine terchloride, or of using toxin which has been 

 acted on with iodine terchloride or with iodine itself. Tizzoni 

 and Cattani have also used the method of administering pro- 

 gressively increasing doses of living cultures attenuated in 

 various ways, e.g. by heat. By any of these methods susceptible 

 animals can be made to acquire great immunity, not only 

 against many times the fatal dose of tetanic toxin, but also 

 against injections of the living bacilli. The degree of immunisa- 

 tion acquired by an animal remains in existence for a very long 

 time. Not only so, but when a high degree of immunity has 

 been produced by prolonged treatment, it is found that the 

 serum of immune animals possesses the capacity, when injected 

 into animals susceptible to the disease, of protecting them 

 against a subsequent infection with a fatal dose of tetanus 

 bacilli or toxin. Further, if injected subsequently to such 

 infection, the serum can in certain cases prevent a fatal result, 

 even when symptoms have begun to appear. The degree of 

 success attained depends, however, on the shortness of the time 

 which has elapsed between the infection with the bacilli or toxin 

 and the injection of the serum. In animals where symptoms 

 have fully manifested themselves only a small proportion of 

 cases can be saved. As with other antitoxins there is no 

 evidence that the antitetanic serum has any detrimental effect 

 on the bacilli. It only neutralises the effects of the toxin. 

 The standardisation of the antitetanic serum is of the highest 

 importance. Behring recommends that for protecting animals 

 a serum should be obtained of which one gramme will protect 

 1,000,000 grammes weight of mice against the minimum fatal 

 dose of the bacillus or toxin. A mouse weighing twenty 

 grammes would thus require "00002 gramme of the serum to 

 protect it against the minimum lethal dose. In the injection 

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