THE DEFENSES OF THE BODY 263 



quired immunity" implies, therefore, a condition of the 

 tissues of an individual, not of necessity peculiar to other 

 members of the race or species, that has originated during 

 his life from the stimulation of his integral cells by one or 

 another of the specific infective irritants that may have 

 been purposely introduced, or accidentally gained access 

 to his body. Acquired immunity may be either active or 

 passive in character. 



Active Immunity. — ^Active immunity is that seen after 

 recovery from infection acquired in a natural way, or from 

 infection induced by the injection of dead or living organisms 

 or the poisons peculiar to them. 



Passive Immunity. — Passive immunity is that condition 

 in which protective substances that have been generated in 

 a susceptible animal by one or the other methods of active 

 immunization are transferred directly from that animal to 

 a normal animal by the injection of the blood serum of the 

 former into the tissues of the latter; the latter being as a 

 rule at once protected. The antitoxic serums have been 

 employed most frequently to bring about passive immunity. 

 The protective value of diphtheria antitoxin in those that 

 have been exposed to infection is well established. The use 

 of tetanus antitoxin for prophylactic purposes is also recom- 

 mended in cases where there is a possibility of the develop- 

 ment of tetanus. 



Vaccination Against Bacterial Diseases. — ^The employment 

 of various prophylactic vaccinations against infectious dis- 

 eases has received much attention in recent years. The 

 measures employed in different diseases vary somewhat, 

 though in general the principles are similar. 



The first measures of this nature that were employed on 

 a large scale are those of HafFkine in vaccination against 



