468 IMMUNITY 



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 toxin or a powerful toxin modified by certain methods. 

 Such methods are the addition to the toxin 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 toxins weakened to varying degrees by being 

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

 In the case of large animals immunisation is sometimes started 

 with small doses of unaltered toxin ; and the doses are gradu- 

 ally increased. The toxin is at first injected into the sub- 

 cutaneous tissues, later into a vein. Ultimately 300 c.c., or 

 more, of active diphtheria toxin 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 immunising the general health of the animal ought not 

 to suffer. If the process is pushed too rapidly the antitoxic 

 power of the serum may diminish instead of increasing, and a 

 condition of marasmus may set in and may even lead to the 

 death of the animal (v. p. 494). (In immunisation of small 

 animals an indication of their general condition may be obtained 

 by weighing them from time to time.) 



4. Estimating the Antitoxic Power of, or "standardising," the 

 Serum. This is done by testing the effect of various quantities 

 of the serum of the immunised animal against a certain amount 

 of toxin. Various standards have been used, of which the two 

 chief are that of Ehrlich and that of Roux. Ehrlich has 

 adopted as the immunity unit the amount of antitoxic serum 

 which will neutralise 100 times the minimum lethal dose of 

 toxin, serum and toxin being mixed together, diluted up to 

 4 c.c. and injected subcutaneously into a guinea-pig of 250 

 grms. weight, the prevention of the death of the animal within 

 four days being taken as the indication of neutralisation, As a 

 standard in testing, Ehrlich employs quantities of serum of 

 known antitoxic power in a dry condition, preserved in a vacuum 

 in a cool place, and in the absence of light. A thoroughly dry 

 condition is ensured by having the glass bulb containing the 

 dried serum connected with another bulb containing anhydrous 

 phosphoric acid. With such a standard test-serum any newly 

 prepared serum can readily be compared. A " normal " antitoxic 

 serum is one of which 1 c.c. contains an immunity unit. 1 c.c. 

 of a serum, of which '02 c.c. will protect from a hundred times 



