472 IMMUNITY. 



the death of the animal. (In immunisation of small animals an 

 indication of their general condition may be obtained by weigh- 

 ing 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. A "normal" antitoxic serum is one of which 

 I c.c. contains an immunity unit. Owing to the difficulty of 

 estimating the occurrence of local infiltration at the site of 

 injection, the prevention of the death of the animal within four 

 days is used as the sole indication of neutralisation, — death 

 later or loss of weight and local infiltration being neglected. 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. Thus i c.c. of a serum of which .02 c.c. will 

 protect from 100 times the lethal dose, will possess 50 immunity 

 units, and 20 c.c. of this serum 1000 immunity units. Sera have 

 been prepared of which i c.c. has the value of 800 units or even 

 more. 



Roux adopts a standard which represents the animal weight in grammes 

 protected by i 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, i c.c. (i grm.) will protect 50,000 grms. of 

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



During the process of development of antitoxin a small 

 quantity of the blood of the animal is withdrawn from time to 

 time, and the antitoxic power tested in the manner described 

 above. After a sufficiently high degree of antitoxic power has 

 been reached the animal is bled under aseptic precautions, and 

 the serum is allowed to separate in the usual manner. It is then 

 ready for use, but some weak antiseptic, such as .5 per cent 



