PASSIVE IMMUNITY 113 



(6) By growth at normal temperatures. 



(c) By growing in the presence of oxygen or air. 



(d) By frequent and prolonged sub-culturing. 



(e) By growth in the presence of very weak anti- 

 septics, or by the injection of an antiseptic with the 

 organisms. 



Passive Immunity. — This condition may be attained when 

 the serum of an animal which has been immunised by one 

 of the previous methods is injected into another animal. 

 This is due to the fact that the serum of a protected 

 animal has a very powerful neutralising or antagonistic 

 effect upon the virulent bacilli if injected at the same time 

 or shortly afterwards. With some infections high resistance 

 can be obtained in certain animals by repeated and in- 

 creasing doses of the toxines obtained from broth cultures 

 of the organism ; the serum of animals so treated will pro- 

 tect other animals against lethal doses of the toxines, or of the 

 virulent organisms themselves. This serum also exercises 

 a protective action against a future infection, although the 

 immunity thus conferred lasts but a short time. Thus 

 these sera become very valuable curative agents, and are 

 the basis of the modern system of serum- therapeutics. 

 The serum of an animal highly immunised against a par- 

 ticular toxine is properly known as ' antitoxic serum '; that 

 of an animal highly protected against a particular organism 

 in a virulent condition is known as ' antimicrobic serum.' 

 This method of inducing immunity was first worked out in 

 the case of diphtheria and tetanus, and has since been 

 appKed with varying success to the treatment of several 

 diseases. 



A combination of the above methods has been success- 

 fully employed to immunise animals — for instance, by 

 repeated injections of cultures, first attenuated and after- 

 wards of very high virulence, a high degree of immunity 



8 



