524 IMMUNITY 



with small doses of unaltered toxin ; and the doses are gradually 

 increased. The toxin is at first injected into the subcutaneous 

 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. The antitoxin content of 

 the serum is estimated from time to time, the object being, of 

 course, to raise it to as high a figure as possible. It is found 

 that each injection produces a certain amount of fall in the anti- 

 toxin value, and this, in favourable cases, is followed by a rise to 

 a higher, level than before, the former event being due in part 

 to the combination of a portion of the antitoxin with the toxin 

 introduced. (Similar phenomena are observed in the develop- 

 ment of all other classes of anti-substances.) 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. After a sufficiently high degree of antitoxic power 

 has been reached, the animal is bled under aseptic pre- 

 cautions, 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 carbolic acid, is usually added to prevent its 

 decomposing. Other antitoxic sera are prepared in a correspond- 

 ing manner. Some further facts about antitetanic serum are 

 given on p. 429. (In immunisation of small animals an indica- 

 tion of their general condition may be obtained by weighing 

 them from time to time.) 



4. Estimating the Antitoxic Power q/, 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. 1 c.c. of a serum, of 

 which '02 c.c. will protect against a hundred times the lethal 

 dose, will possess 50 immunity units, and 20 c.c. of this serum 

 1000 imm unity 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 



