INTRODUCTORY AND GENERAL 27 



at once and without a negative phase. Unfortunately, it is not 

 always or often possible. 



2. As regards its duration. Active immunity lasts for a long 

 time, the length differing greatly in different diseases and after 

 various methods of induction. In many cases it lasts a year or 

 more. Passive immunity, on the other hand, is always of brief 

 duration, and lasts only about as long as the serum injected is 



a 



FIG. 2. SHOWING THE SEQUENCE OF EVENTS IN THE PRODUCTION OF 

 ACTIVE IMMUNITY. 



An injection of vaccine at b is followed by a decrease in the degree of 

 immunity (negative phase), a rise, and a gradual return to the normal 

 condition. 



actually present in the blood. It depends to a certain extent on 

 the dose of serum given, and also on the species of animal from 

 which it was derived. An animal of a certain species is immu- 

 nized for a longer period by serum from another animal of the 

 same kind than from one of a different species. In general terms 

 the duration of passive immunity is three to six weeks. It is 

 renewable at pleasure, as far as we know indefinitely. 



FIG. 3. SHOWING THE SEQUENCE OF EVENTS IN PASSIVE IMMUNITY. 

 An injection of serum is given at b. 



Hence passive immunity is chiefly of value to ward off an 

 infection the danger to which is of short duration. Thus in 

 veterinary practice the passive immunity of horses conferred by 

 the injection of tetanus antitoxin is of the greatest possible value 

 before operations, or immediately after the infliction of a wound, 

 horses being so prone to tetanus that in some places any opera- 



