270 INFECTION AND RESISTANCE 



For the purpose of demonstrating this analogy two protected 

 solutions were prepared as follows : 



Solution 1. This consisted of 2 drops of gum arabic, 2 c. c. of gelatin, and 

 5 c. c. of the weaker arsenic solution. 



Solution 2. This consisted of 10 drops of gum arabic, 1 c. c. of gelatiD, 

 and about 4 c. c. of the stronger arsenic solution. 



In each case the arsenic snlphid was added until there were signs 

 of increasing opalescence or turbidity, this being done in order that 

 the two solutions should each be as little overprotected as possible. 



Portions of the two solutions were then mixed in equal propor- 

 tions. In the course of a few minutes the mixture was noticeably 

 more turbid than either of the original solutions. This turbidity 

 continued to increase quite rapidly, and on the following morning 

 after about sixteen hours of standing, the mixture was found to b 

 completely flocculated out, while the original protected mixtures re- 

 mained unprecipitated and showed about the same degree of opales- 

 cence as on the preceding night. The same condition of affairs was 

 found to have persisted after five days. On the fifth day the less 

 concentrated of the clear protected suspension began to settle out, 

 and was completely precipitated within twenty-four hours. The 

 other remained clear for four days more, but on the ninth day it 

 began to precipitate slightly, the precipitation remaining incom- 

 plete. 



In these cases it appears, therefore, that a complete analogy to 

 the observed conditions of the serum reactions has been found, and 

 that all data observed in connection with sera in which antigen and 

 precipitin are found side by side without reacting can be most simply 

 explained on the conception of protective colloid action. Moreover, 

 the chemical nature of the substances involved seems to add weight 

 to this point of view. 



These relations have been gone into here at some length, since 

 they seem to us to possess considerable theoretical and practical sig- 

 nificance. For it may be that the presence of a protective colloid 

 may, by inhibiting the union of antigen and precipitin within the 

 body, protect the animal from intoxication during the early stages 

 of immunization when antigen and antibody are present simulta- 

 neously for longer or shorter periods. Were union between the two 

 possible at such times in the circulation, an assumption necessitated 

 both by the hypotheses of mass action and of multiplicity of precip- 

 itins, there would probably be an absorption of complement by these 

 complexes, with, as shown by Friedberger, a consequent formation of 

 powerful toxic products. (See chapter on Anaphylaxis.) It is 

 not impossible by any means, therefore, that the injection of anti- 

 gen in an animal in which such a balance has been established may 



