IMMUNITY TO TOXINS 125 



cytes, and the continuance of the factors by which the toxin is 

 combated in the first stage of the infection. 



Passive antitoxic immunity consists in the artificial production 

 of this stage. 



3. In the third stage, which is only reached in hyperimmunized 

 animals submitted to treatment for long periods, the conditions 

 are quite different, and do not appear to depend in any way on the 

 action of antitoxin, but approach more nearly to the state of 

 natural immunity to be treated subsequently. In this condition the 

 antitoxin in the blood falls greatly, and would probably disappear 

 entirely if the treatment were carried further, yet the degree of 

 immunity is very great. There are two or three theories which 

 have been invoked to explain this form. 



In the first place, if we accept the side-chain theory, we may 

 ascribe it to the absence of receptors suitable for the toxin which 

 is being injected, and may suppose that the repeated and pro- 

 longed stimulation of the production of this particular receptor 

 has led firstly to a hypertrophy, and secondly to an atrophy of 

 these organs. This is readily conceivable, and might be compared 

 with the sequence of hypertrophy and failure of the heart so 

 frequently met with in Bright's disease, etc. In this case the 

 toxin would be unable to " anchor " itself to the cells, which would 

 thus escape its action, and the result would be one form of tissue 

 immunity. There is but little experimental evidence bearing 

 directly on this theory either for or against. The state is one 

 rarely seen even under experimental conditions, though of great 

 theoretical interest. It is found, however, that during the immuni- 

 zation of the rabbit with eel serum (a potent haemolytic agent) an 

 antitoxin or antihaemolysin is produced, whereas the corpuscles 

 themselves, if carefully washed from all traces of serum, are as 

 susceptible as before. If, however, the injections are repeated, 

 the antitoxin disappears, and, as Tchistovitch showed, about this 

 time the red corpuscles themselves become immune, not being 

 haemolyzed by eel serum, even although all traces of their own 

 serum is washed out. This we may fairly suppose to be due to a 

 loss of the receptors with which the molecules of eel serum 

 combine, and this may be taken as an experimental demonstration 

 of the occurrence of this form of immunity, which was predicted 

 by Ehrlich on theoretical grounds. Metchnikoff, it is true, 

 objects to this hypothesis, pointing out that if the receptors are so 

 necessary for the nutrition of the molecule it is impossible that 



