564 IMMUNITY 



1000 immunity units. Sera have been prepared of which 1 c.c. 

 has the value of 800 units or even more. 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. 



Roux has adopted a standard which represents the animal weight in 

 grammes protected by 1 c.c. of serum against the dose of virulent bacilli 

 lethal to a control guinea-pig in thirty hours, the serum being injected 

 twelve hours previously. Thus, if - 01 c.c. of a serum will protect a 

 guinea-pig of 500 grms. against the lethal dose,- 1 c.c. (1 grin.) will pro- 

 tect 50,000 grms. of guinea-pig, and the value of the serum will be 50,000. 



Sera of Animals immunised against Vegetable and Animal Poisons.— 

 It was found by Ehrlich in the case of the vegetable toxins, ricin and 

 abrin, and also by Oalmette and Fraser in the case of the snake poisons, 

 that the serum of animals immunised against these respective substances 

 had a protective effect when injected along with them into other animals. 

 Ehrlich found, for example, that the serum of a mouse which had been . 

 highly immunised against ricin by feeding as described above, could 

 protect another mouse against forty times the fatal dose of that substance. 

 He considered that in the case of the two poisons, antagonistic substances 

 — "anti-ricin" and "anti-abrin" — were developed in the blood of the 

 highly immunised animals. A corresponding antagonistic body, to 

 which Fraser gave the name "antivenin," appears in the blood of 

 animals in the process of immunisation against snake poison. 



These investigations are specially instructive, as such vegetable and 

 animal poisons, both as regards their local action and the general toxic 

 phenomena produced by them, present, as we have seen, an analogy to 

 various toxins of bacteria. 



Nature of Antitoxic Action. — This subject is only part of the 

 general question with regard to the relation of anti-substances 

 to their corresponding antigens, but it is with regard to anti- 

 toxic action that most of the work has been done. We have to 

 consider here two points, namely, (a) the relation of antitoxin 

 to toxin, and (b) the source of the antitoxin. With regard to the 

 former subject there is now no doubt that the antagonism 

 between toxin and antitoxin is not a" physiological one, but that 

 the two bodies unite in vitro to form a compound inert towards 

 the living tissues, there being in the toxin molecule an atom 

 group which has a specific affinity for the antitoxin molecule or 

 part of it. 



When toxin and antitoxin are brought together in vitro, it 

 can be proved that their behaviour towards each other resembles 

 what is observed in chemical union. Thus it has been found 



