80 STUDIES IN IMMUNITY. 



of destroying the vibrio, when mixed with normal serum, which in 

 itself is only faintly bactericidal, acquires new and energetic destruc- 

 tive properties. It is not necessary to imagine any reaction on the 

 part of cells, due to preventive serum, to form this normal substance 

 contributed by the normal serum, since it has been shown to be 

 regularly present in the normal animal. This substance is not 

 uniformly dissolved in the plasma during life, but is confined rather 

 within the leucocytes. 



What happens, then, when preventive serum is inoculated? The 

 active substances penetrate into the leucocytes as we already know 

 by the fact that leucocytes show evidence of chemiotaxis in presence 

 of this serum. The preventive substance finds in the leucocytes 

 the bactericidal substance there present. From this moment the 

 leucocyte has acquired a powerful and specific bactericidal prop- 

 erty. If it takes up vibrios it can destroy them ; but if, for any 

 reason, it should suffer injury, it will liberate the bactericidal sub- 

 stance into the surrounding fluid and so bring about an extra- 

 cellular destruction of the vibrio. If we consider the facts from 

 this point of view, as seems justifiable, we must admit that in 

 addition to cellular stimulation there is present a purely chemical 

 phenomenon that may be repeated in vitro and which is of great 

 importance in passive immunity. This phenomenon is the union of 

 the two substances that produce a bactericidal power either in the 

 animal or in the test tube. An immunity of this sort brought about 

 in this manner need not be very powerful, and it is not necessary 

 for its formation that a profound cellular change should be caused 

 as would be essential for its persistence. Passive immunity is 

 essentially ephemeral and evidence of it is not long shown by 

 the cell. As soon as the preventive substance is eliminated the 

 leucocytes will have no more than a limited bactericidal property, 

 and the refractory condition so rapidly obtained will be quite as 

 rapidly lost. 



