PASSIVE IMMUNITY. 



169 



of the technic evolved by Leishman and by 

 Wright, this principle has been found to have a 

 wide, almost universal application : in other words, 

 active immunization, as in a natural infection or 

 by the injection of bacterial cells, is almost invari- 

 ably accompanied by an increase in opsonins, which 

 appears to coincide with an increase in the phago- 

 cytic power of the blood. 



Inasmuch as it has proved possible by the pro- 

 longed immunization of animals with bacteria or 

 toxins to induce a high concentration of antibac- 

 terial or antitoxic substances in their serum, it 

 was the natural expectation that if such serums 

 were injected into other animals the latter would 

 thereby be endowed with an increased resistance 

 to the infectious agent against which the serum 

 had special activities (passive immunization). 

 This has been found to be the case with many 

 antibacterial (typhoid, cholera, plague, dysentery, 

 etc.) and some antitoxic serums (diphtheria, teta- 

 nus). Unfortunately the protection afforded by 

 the injection of an immune serum is of short dura- 

 tion (from two to several weeks) ; it is as if a for- 

 eign substance had been injected, the fate of which 

 is to be eliminated rapidly. This is in contrast 

 to the condition in active immunity, in which the 

 protective substances are often formed over a long 

 period by the body cells. 



The school of Metchnikoff brings the leucocytes 

 into relation with passive as well as active im- 

 munity. It is held that the immune serum which 

 is injected is potent, because it stimulates the leu- 

 cocytes to a greater phagocytic activity in the case 

 of antibacterial immunity, or to a greater absorp- 



Passive 

 Immunity. 



The Leucocytes 



