THE PHENOMENA OF ANTITOXIN FORMATION 67 



In the earlier stages of immunization, as we have seen, each 

 injection is followed (after a negative phase) by a rise in the anti- 

 toxic value of the serum above its previous level. In the stage 

 which now follows this does not occur, or not definitely ; there is 

 a negative phase, but it is found impossible to force the antitoxic 

 value above a certain level, which varies in different horses. 

 This second stage, or period of maintained maximum, varies in 

 different horses, and may last a few months or a year. While it 

 lasts there are, of course, oscillations ; it falls, for instance, if the 

 animal contracts any disease or suffers in general health, but its 

 general average is about the same. 



Sooner or later this state of affairs changes, and the antitoxic 

 value of the serum begins to fall, and cannot be raised or even 



FIG. 7. 



a, b, Normal resisting power ; b, c, period of hypersensitiveness ; c, d, period 

 of rise in immunity ; and d, e, maintained high level thereof. /, g, Normal 

 amount of antitoxin ; g, h, period in which it increases ; and h, i, gradual 

 fall and ultimate (theoretical) disappearance. 



maintained at its former level in spite of very large doses of toxin. 

 It trends steadily downward, although the animal may continue 

 to give useful serum for a long time. Thus in one of Atkinson's 

 best horses (out of a series of 100) the serum contained 1,000 to 

 1,100 antitoxic units per cubic centimetre for ten months, and 

 then gradually sank, but remained above 300 units per cubic 

 centimetre for two years. 



After a prolonged rest the power of manufacturing antitoxin 

 may return, but then only lasts a short time, and cannot be re- 

 newed again. 



The third stage, therefore, consists in a gradual disappearance 

 of antitoxin from the blood, without any loss of the immunity to the 

 toxin. It would seem, indeed, as if the immunity reaches its 

 highest level at this point, in spite of the almost complete absence 



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