102 ACTIVITIES OF BACTERIA. 



the phenomena may be represented, according to Ehrlich, 

 somewhat as follows: If, with the first cautious intro- 

 duction of a small amount of tetanus poison only a small 

 part of the toxophoric side chains is fixed, then only a 

 few cells are injured, or many cells are so slightly injured 

 that no disease symptoms occur. The side chains united 

 with toxin are thrown off and replaced by the cells. This 

 repair is always accomplished more rapidly, the more fre- 

 quently and extensively the side chains are torn off by the 

 toxins. Finally, the cells produce more side chains than 

 can remain connected with them, and they are discharged 

 into the blood as antitoxin. Knorr finds this explanation 

 of the facts unsatisfactory, and claims that, while in one 

 part of the diseased organism the antitoxin groups are 

 fixed by toxin, the unpoisoned cells containing antitoxin 

 are stimulated to a larger production of this substance. 



From this essential independence of antitoxin from the 

 chemical composition of the toxin one can understand that 

 a specificity of antitoxin does not strictly exist. For ex- 

 ample, tetanus and rabies antitoxin also protect against 

 cobra poison (Calmette, A. P. ix, p. 225), and, according 

 to Tizzoni, sterile non-toxic cultures of pneumococcus in 

 rabbits' blood protect against tetanus poison (C. B. xxiv, 

 904). 



The value of antitoxic sera is expressed as follows: 



1. After Behring. He designates as a normal poison a 

 toxin solution of which 0.01 c.c. is sufficient to kill a 

 guinea-pig weighing 250 gm. within four days. The toxin 

 is always injected in 4 c.c. of water just beneath the skin. 

 Of this normal diphtheria toxin (DTN) 1 c.c. is sufficient 

 to kill one hundred guinea-pigs, each weighing 250 gm. , 

 or 25,000 gm. in weight of guinea-pigs. Behring ex- 

 presses this as follows: 



The DTN has a working value of 25,000; it kills 25,000 

 times its weight of guinea-pigs. A toxin ten times as 

 strong is represented as DTN 1 ; a ten times weaker one, as 

 DTN 

 10 . 



The quantity of antitoxin that is required to just pro- 

 tect 25,000 gm. weight of guinea-pigs from the minimal 

 fatal dose of poison is called one immunizing unit. 



