INTRODUCTION 5 



identical with opsonins and may be increased by active 

 immunization. 



Acquired Immunity, — Acquired immunity is that 

 condition of protection against disease, resulting from 

 recovery from infection or arising by virtue of arti- 

 ficial inoculation. It may be produced in two ways, 

 namely, by active or passive immunization. 



Active immunization signifies the process by which 

 the bodily cells of an animal are sti7nulated by a toocin 

 or foreign body {antigen) to the production of cer- 

 tain other bodies {antibodies) specific against the 

 given foreign substance (see Chapter II). Thus the 

 animal is actively concerned in the elaboration of its 

 own antibodies, hence the process is termed active. 

 Untreated disease terminates in either one of three 

 ways — death, recovery, or chronicity. If the viru- 

 lence of the infection is great and the dose large or 

 overwhelming, the animal succumbs, especially if his 

 vital resistance be slight. If the infection is relatively 

 avirulent, even though the dose be large, recovery al- 

 though protracted may take place provided the ani- 

 mal's resistance is great. If the degree of virulence of 

 the infection be merely the average, but the vital resis- 

 tance of the animal be only mediocre, its cellular ac- 

 tivity or infective dose may prove inadequate for the 

 normal generation of sufficient specific antibodies to 

 insure immunity, and chronic invalidism results. Thus 



