226 STUDIES IN IMMUNITY. 



any one of these sera endows it with curative or preventive proper- 

 ties for normal animals. We may say simply that the function of 

 sensitizers in protecting animals must vary with the bacteria under 

 consideration. Certain of these bacteria are easily destroyed in 

 fixing alexin, others under the same conditions resist better (leaving 

 aside any purely physiological alteration that may occur); and 

 others doubtless absorb alexin with impunity.* Several therapeu- 

 tic sera owe their activity in great part to the presence of anti- 

 toxins. The seat of infection must enter into account, as alexin is 

 not uniformly distributed throughout the body. It is certain that 

 the part played by each substance of active serum in a cure by 

 serum therapy must vary with the infection under consideration. 



There is one fact frequently mentioned in this article that we 

 may consider for a moment. The bacteria studied had little or 

 no alexin-fixing property unless sensitized, and even when sensitized 

 the bacteria had to be relatively numerous to absorb alexin. Con- 

 sequently, in animals invaded by a pathogenic micro-organism, we 

 should scarcely imagine death to be due to an insufficiency of alexin 

 in the body. An assertion that this is the cause of death is, more- 

 over, to lose sight of the fundamental and well-proved idea that 

 yielding to an infection is primarily due to an inability of the phag- 

 ocytes to take up the infective agent. And then even supposing 

 protection by alexin to be the essential factor in immunity death 

 would not be due to a lack of alexin, but rather to a lack of utilizing 

 or absorbing it. 



We should scarcely expect in treating human bacterial infections 

 that as Wassermann has expressed the hope the administra- 

 tion of certain normal sera or alexins in addition to the specific serum 

 could be recommended. It is unreasonable, since the alexins from 

 foreign species affect not only bacteria, but body cells. And, more- 

 over, the animal would soon protect itself from such injections by 

 forming anti-alexins. 



* We intend to determine whether the serum of tuberculous individuals con- 

 tains a sensitizer capable of causing Koch's bacillus to absorb alexin. If such a 

 sensitizer is present, it would be a good example of one that serves no distinctly 

 useful purpose. 



