ANTITOXIC SERUM. 471 



"normal diphtheria toxine of simple strength" (DTN 1 ), 

 as indicating a toxine of which .01 c.c. is the minimum 

 lethal dose under these conditions. A toxine of which the 

 minimum lethal dose is .02 will be of half normal strength 

 (DTN <5 ); and so on. The testing of a toxine directly 

 is a tedious process, and in actual practice, where many 

 toxines have to be dealt with, it is found more convenient 

 to test them by finding how much will be neutralised by 

 a certain amount of a standard antitoxic serum, viz., an 

 "immunity unit." 



3. Development of Antitoxine. The earlier experiments 

 on tetanus and diphtheria were performed on the small 

 animals, such as guinea-pigs, but afterwards the sheep and 

 the goat were used, and finally horses. In the case of the 

 small animals it was found advisable to use in the first 

 stages of the process either a weak toxine or a powerful 

 toxine modified by certain methods. Such methods are 

 the addition to the toxine of terchloride of iodine (Behring 

 and Kitasato), the addition of Gram's iodine solution in the 

 proportion of one to three (Roux and Vaillard), and the 

 plan, adopted by Vaillard in the case of tetanus, of using 

 a series of toxines weakened to varying degrees by being 

 exposed to different temperatures, viz. 60, 55, and 50 C. 

 But in the case of large animals, such as the horse, the first 

 injections are simply made with small doses of the ordinary 

 toxine. The toxine is at first injected into the subcutaneous 

 tissues, the dose being gradually increased according to the 

 results of the toxine injected. As pointed out by Behring, 

 immunisation proceeds best when each injection produces a 

 reaction in the form of localised inflammatory swelling ; in 

 other words, the dose should be as large as possible, so long 

 as general injurious effects are not produced. Later, when 

 large doses of toxine injected subcutaneously are well borne, 

 the toxine is injected directly into the jugular vein of the 

 animal. Ultimately 300 c.c., or more, of active diphtheria 

 toxine thus injected may be borne by a horse, such a degree 

 of resistance being developed after the treatment has been 

 carried out for two or three months. In all cases of 



