42 ACTIONS OF TOXINS 



was treating for the production of diphtheria antitoxin developed 

 tetanus, and tetanus toxin was found in a sample of blood collected 

 five days before the development of symptoms. 



On diminishing the amount of the toxin which we inject, we 

 find that the latent period becomes gradually longer, and the 

 duration of the disease (i.e., the time between the first develop- 

 ment of symptoms of intoxication and the fatal issue) also lengthens. 

 By diminishing the dose gradually we can find an amount which 

 will just kill the animal in question in a given number of days, 

 and, provided the test animals used are approximately the same 

 in age and weight, we shall find that this amount, the " minimal 

 lethal dose," is fairly constant for animals of the same species. 

 Thus, in the standardization of diphtheria antitoxin the first step 

 is the estimation of the minimal lethal dose of the toxin, and for 

 this purpose it is customary to use guinea-pigs weighing from 

 250 to 280 grammes, and to fix a time-limit of four days. It is 

 found that the minimum lethal dose is the same, within close 

 limits, for all test animals, and that if a series similar in size and 

 weight be inoculated with the same dose, the majority will die 

 within a few hours of one another. This fact enables diphtheria 

 antitoxin to be titrated with some approach to chemical accuracy, 

 the test guinea-pig being used as the indicator. 



On reducing the dose still further, we find that the incubation 

 period is still further prolonged, that the symptoms are less severe, 

 and that death may not take place, or only do so at a later period 

 than that which has been fixed for the minimal lethal dose. Thus, 

 in antitoxin-testing a dose of toxin which does not kill in five days 

 is regarded as a sublethal dose, although death may take place at 

 a later date perhaps much later. 



On giving still smaller doses the symptoms take still longer to 

 develop, are still slighter, and are followed by recovery, and the 

 animal may then present a certain degree of immunity to the toxin 

 and to the organism producing it. On the other hand, under 

 certain circumstances it may be more than usually sensitive to 

 the action of the toxin in question. 



These phenomena present some points of comparison with 

 those which are presented in the action of the soluble enzymes, 

 such as pepsin. In each case an excessively minute amount of 

 the active substance will produce the given effect, and in each 

 the effect is more rapid if a larger amount be used. In either 

 case there is a latent period of longer or shorter duration before 



