174 INFECTION AND IMMUNITY. 



poisons whose virulence has been in one way or another reduced but not 

 rendered altogether inert ; or by the action on relatively insusceptible animals 

 of microbes or microbic poisons of unimpaired virulence. Immunity induced 

 in this way as the result of direct adaptation of the body cells to the new con- 

 ditions is called ACTIVE IMMUNITY. 



1. Insusceptibility to particular forms of infectious disease may be conferred by 

 inoculation with cultures of the germ inciting the infection whose virulence has been artifi- 

 cially reduced. This reduction of virulence of the micro-organisms may be accomplished 

 in various ways by cultivation at temperatures above their optimum; by successive 

 inoculations into insusceptible animals; by prolonged artificial cultivation in the pres- 

 ence of oxygen ; by exposure to certain inorganic chemical substances, as the diphtheria 

 bacillus to trichlorid of iodin, anthrax to bichromate of potash, etc. ; by exposure of 

 cultures to organic extracts or products of animal or vegetable cell metabolism ; by 

 drying (hydrophobia), or by exposure to sunlight ; and in other ways. 



With the virulence of the micro-organisms reduced in varying degrees in one or 

 other of the ways just mentioned, the gradual habituation of the bodies of animals to 

 the presence of pathogenic germs may be pursued until cultures of full virulence are 

 tolerated. 



2. Immunity may be conferred by the injection, in gradually increasing doses, of 

 the metabolic products of bacterial growth, either with or without the dead bodies of the 

 germs themselves the bacilli of typhoid fever for example. The primary virulence of 

 these usually toxic products of microbic growth may be in various ways diminished, 

 by heating, by mixing with organic extracts such as that of the thymus gland, or with 

 an inorganic chemical substance such as trichlorid of iodin, or by small doses of the 

 already prepared antitoxin see below. 



3. Immunity may be secured in some cases by the inoculation of animals, which 

 are but moderately susceptible to the species employed, with small but increasing quan- 

 tities of germs having unimpaired virulence. Under these conditions the animal becomes 

 less and less responsive to the germ, until finally it may display no reaction after a 

 quantity of the virulent culture which at first would have been inevitably fatal. 



Immunization in man by the direct use of microbes or microbic poisons or virus of 

 diminished virulence has been largely practised in typhoid fever, cholera, and hydro- 

 phobia. 



In active immunity the protective material is elaborated by the im- 

 munized individual, the process requires considerable time, and the eft'ect 

 lasts for a considerable period. 



II. In a second class of procedures artificial immunity is secured by the 

 direct mingling of the body fluids from an individual already immunized in 

 some of the above ways with those of the individual to be protected. Immunity 

 secured in this way in which the immunizing substance used has been elaborated 

 by another individual is called PASSIVE IMMUNITY. 



1. Extracts of various organs and tissues of animals suffering from infectious disease 

 rendered germ-free and injected into healthy animals, have been found in some cases to 

 confer a certain degree of immunity. 



2. The blood serum of animals naturally immune to a particular infectious disease 

 has been found on injection into those which are susceptible to the same disease to im- 

 part in some cases a certain degree of insusceptibility. 



3. The blood serum finally of animals which have been rendered in one way or 

 another artificially immune to certain diseases, if introduced under proper conditions 

 into another susceptible animal, has been found not only to confer a temporary immu- 

 nity, but if administered to an already stricken individual to aid him in the most marked 

 and efficient way to overcome the deleterious agencies at work. 



