374 Chapter XII 



serum oscillated between 200 and 300 units. After five years of this 

 state of things the antitoxic power began to fall considerably, with- 

 out, however, any corresponding loss of immunity. Indeed, an 

 injection of 250 c.c. of toxin (of which 0'002 c.c. was sufficient to kill 

 a guinea-pig) began, at the commencement of the present year, to be 

 borne without the least febrile reaction. An attempt was made to 

 raise the antitoxic power of the blood by making intravenous in- 

 jections of toxin and of diphtheria culture, but in vain. The yield of 

 antitoxin continued to fall and it became necessary to employ this 

 horse for another purpose than the preparation of antidiphtheria 

 serum. This is by no means an isolated example. Of a large number 

 [393] of treated horses it frequently happens that certain individuals, with- 

 out being particularly susceptible to a given toxin, are found to be 

 incapable of producing any corresponding antitoxin 1 . 



In presence of the fact that animals very resistant to toxins may 

 possess no, or only an insignificant antitoxic power in their fluids, 

 and that, on the other hand, animals in which this property is 

 highly developed may succumb to intoxication, it may be readily 

 understood that immunity against toxins and the antitoxic power 

 of the body fluids may be two distinct conditions. Von Behring 

 has clearly demonstrated the fact of the cellular hypersensitive- 

 ness of the animal immunised against the corresponding toxin 

 and has laid great stress upon this fact. He came 2 to the con- 

 clusion that "the immunity of the tissues and the production of 

 antitoxin follow a parallel course in their development so slightly 

 that, in spite of an abundant accumulation of antitoxin, the suscepti- 

 bility of the elements of the tissues may increase in an extraordinary 

 fashion." If, during the course of immunisation, this susceptibility 

 can increase so greatly, it is probable a priori that under certain 

 circumstances it might also diminish notably. After demonstrating 

 "that in time the antitoxin disappears from the blood of animals 

 immunised with toxins without any consequent disappearance of 

 immunity," von Behring formulated the conclusion that in these 

 animals " the living elements of the animal, which were previously 

 susceptible to the poisons, have acquired an insusceptibility towards 

 the same substances." This result fully accords with the facts of the 

 change of the negative chemiotaxis of phagocytes into positive 



1 These observations were communicated to me by M. Prevot, the director of the 

 erotherapeutic station of the Pasteur Institute at Garches. 



2 Deutsche nied. Wchnschr., Leipzig, 1893, SS. 1253, 1254. 



