INFECTION AND IMMUNITY. 575 



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. 



Active Immunity. Acquired immunity may be either 

 active or passive in character. Active immunity is 

 that form which results from recovery from infection 

 acquired in a natural way, or from infection induced by 

 the injection of dead or living organisms as a prophy- 

 lactic measure against infection. 



Passive Immunity. Passive immunity is that form 

 in which the immune bodies generated in a susceptible 

 animal, as the result of systematic injections of dead or 

 living cultures, are introduced into a human being 

 to protect against infection. The antitoxic serums have 

 been employed most frequently to bring about passive 

 immunity. The protective value of diphtheria anti- 

 toxin in those that have been exposed to infection is 

 well established. The use of tetanus antitoxin for pro- 

 phylactic purposes is also recommended in cases where 

 there is a possibility of the development of tetanus. 



VACCINATION AGAINST BACTERIAL DISEASES. 

 The employment of various prophylactic measures 

 against infectious diseases 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 the Haffkine vaccination 

 against cholera and plague by means of cultures that 

 had been killed after heating to a moderate temperature. 

 The dead organisms when injected bring about a reac- 



