R£SEARCHiES ON CERTAIN PROBLEMS OF PLAGtJE IMMUNITY 29 



These tests appear to show : — 



1. That the serum is capable of neutralising the toxin when mixed 



in vitro^ as tested by subsequent subcutaneous or intraperi- 

 toneal injection into rats and mice. 



2. That the serum injected into rats or mice, 24 hours before the 



toxin, is capable of conferring protection on these animals. 



3. In the case of rats, 002 c.c. of serum, when mixed with the toxin, 



neutralises the toxic action of about 10 M. L. D., and i c.c. 

 of serum protects against at least 10 M. L. D. injected at 

 another part of the body 24 hours after the serum. 



Kolle says that the serum of the normal horse has considerable 

 antitoxic properties, but, as already stated, he was unable to obtain an 

 increase beyond what he regarded as within the limits of normal 

 variation. 



If the serum of an animal normally contains antitoxin, we might 

 anticipate on the basis of Ehrlich's theory that it would be possible by 

 stimulating the organism to obtain an increase in the production of the 

 antitoxin. In our experiments, for purposes of control, the sera of four 

 normal horses were employed, but, in addition, the serum of five horses 

 immunised by the ordinary method of obtaining anti-plague serum 

 showed little or no increase in antitoxic power, and these may, therefore, 

 be classed among the controls. 



The evidence from these experiments seemed to be in favour of the 

 possibility of obtaining a serum with antitoxic properties in excess of 

 those found in normal serum. 



Relation of Neutralisation of Toxin to Precipitin Reaction. — Certain 

 experiments were carried out with the view of ascertaining whether the 

 occurrence of the precipitin reaction of Kraus might not play some part 

 in the plague toxin-antitoxin reaction. 



The serum of certain of the horses gave a well marked'fprecipitation 

 with the Test Toxin No. 10. The serum of the horse " R D." gave 3 to 4 

 times the precipitum given by the serum of the horse " M." In the 

 previous tests we have seen that whereas i c.c. of the serum of " R D." 

 was required to neutralise 10 M. L. D. of Toxin 10, 0*02 c.c. of the serum 

 of " M." produced the same effect. There is, therefore, no direct pro- 

 portionality between the two properties of the serum. 



iM9) 



