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appearance in the blood after clotting (when fibrin ferment is also 

 liberated from these cells), and their absence from fluids containing 

 none, such as the aqueous humour. 



It was obvious that this was a foundation for a theory of 

 immunity, but it soon became apparent that it did not explain the 

 whole of the phenomena. An animal might be immune to a 

 certain organism, yet its serum might not be bactericidal for it, or 

 it might be susceptible and yet possess suitable alexins. Thus, 

 rabbits are very susceptible to anthrax, yet have serum which 

 kills the bacilli in large numbers, whilst the dog is much less 

 susceptible, though its serum is very slightly bactericidal. Such 

 facts prevented the alexic theory of immunity from making head- 

 way until the discovery of Pfeiffer's phenomenon. 



Pfeiffer found that when cholera vibrios were injected into the 

 peritoneal cavity of a highly immunized guinea-pig they were not 

 only killed, but dissolved, and this with great rapidity. The 

 vibrios are first rendered immotile, and then lose their proper 

 shape, becoming converted into spherical masses which stain 

 badly. In a little while they become smaller, and disappear alto- 

 gether, no trace being left. 



Pfeiffer showed that the phenomenon could also be produced 

 by injecting into the peritoneum of a normal guinea-pig a mixture 

 of serum from an immunized animal and the culture of vibrios. 

 This happened when an old specimen of serum in which the 

 alexins had disappeared spontaneously was used, or a fresh 

 specimen that had been heated to 60 C. Lastly, he found that 

 if an old immune serum were injected into the peritoneal cavity 

 and allowed to remain for a while, it regained its bactericidal 

 powers, and could then dissolve the vibrios in vitro at the body 

 temperature. The phenomenon was specific i.e., if serum from 

 an animal immunized against cholera were used as described 

 above, the vibrios of cholera were dissolved, but no others. If 

 typhoid serum were used, typhoid bacilli showed degenerative 

 changes, though they were not completely destroyed, but there 

 was no effect on cholera vibrios. 



The explanation which suggested itself to Pfeiffer was this : 

 There is in the serum of highly immunized animals a substance 

 which can exist either in an active or inert state. In the blood- 

 serum or peritoneal fluid whilst in the body it occurs as an active 

 substance, but it passes into the inert condition when kept for a 

 few days, or very rapidly when heated to 60 C., and this inactive 



