446 IMMUNITY. 



a small drop of anti- cholera serum ; (c) a drop of this 

 mixture was taken, and there was added to it a drop of 

 equal size of fresh serum from a normal guinea-pig. A 

 hanging-drop preparation was made, and a change similar 

 to that described by Pfeiffer was observed within one to 

 two hours if the preparation was' kept at the temperature 

 of the body. He found that in every case in which 

 Pfeiffer's reaction took place within the body of an animal, 

 a similar reaction could be observed by his method out- 

 side the body. He considers that the reaction depends 

 upon the presence of a specific immunising substance 

 in the anti-serum which greatly increases the bactericidal 

 power of the normal serum, but that in most cases this 

 specific substance cannot lead to the destruction of the 

 organisms without the aid of healthy serum. Bordet 

 holds that the source of the protective substance as well 

 as that of bactericidal substances is in the leucocytes. 



Charrin and Roger had previously (1889) observed 

 that when the bacillus pyocyaneus was grown in the serum 

 of an animal immunised against this organism, the growth 

 formed a deposit at the foot of the vessel ; whereas a 

 growth in normal serum produced a uniform turbidity. 

 Gruber and Durham, in investigating Pfeiffer's reaction, 

 discovered a somewhat analogous phenomenon. They 

 found that when a small quantity of the serum of an animal 

 highly immunised against a particular motile organism 

 (cholera vibrio, typhoid bacillus, etc.) is added to an emulsion 

 of the organisms, the latter lose their motility, and become 

 agglomerated into clumps. In a small test-tube a reaction 

 in this way occurs which is visible to the naked eye, a sort of 

 precipitate forming which consists of masses of non-motile 

 organisms. The higher the degree of immunity, the smaller 

 is the amount of serum necessary to bring about this phe- 

 nomenon. The serum, therefore, has a sort of paralysing 

 action against the organism, which is manifested outside 

 the body, and which Durham considers forms the essen- 

 tial part of Pfeiffer's reaction. When the organisms are 

 thus weakened by the specific serum within the peri- 



