24 THE PRINCIPLES OF IMMUNOLOGY 



spore^bearing organisms. Those organisms which do not produce 

 spores can be killed by heat of 58 to 60 C. for thirty to sixty 

 minutes, and this degree of heat does not alter the character of the 

 proteins. 'The chemicals most frequently employed for killing bac- 

 Iteria so as not to alter the proteins are formaldehyde and phenol. 



Immunization with Bacterial Products. In addition to the use 

 of dead bacteria, as indicated above, it has been found possible to 

 produce immune reactions by the use of extracts of the organisms, 

 these extracts containing a considerable amount of bacterial pro- 

 tein. Immunization of this sort leads to the formation of anti- 

 bacterial sera which agglutinate the bacteria or precipitate bacterial 

 extracts. It is possible also that this method of immunization leads 

 to the formation of other immune substances. How far protein, 

 either in solution or in the bodies of bacteria, may be broken down 

 and still be capable of leading to the formation of immune bodies is a 

 iquestion that has been extensively studied. Certainly any change 

 that breaks up the protein into its fundamental amino-acids is likely 

 to destroy its antigenic properties. Simple fractionation by means 

 of salting still leaves sufficient native protein to serve to immunize. 



Of bacterial products which have been employed for immuniza- 

 tion none is more important than those poisonous bodies called 

 toxins. In the classification of toxins we have referred to the true 

 toxins or exotoxins and to the endotoxins. There is little support 

 for the belief that endotoxins as such, except in rare instances, can 

 produce immune substances. On the other hand, the production of 

 a neutralizing antitoxin against the exotoxins has constituted one 

 of the most brilliant chapters in the study of immunology, and it 

 will be given discussion in the chapter on toxins and antitoxins. 

 The use of toxins as antigens involves the employment of these 

 substances in non-fatal doses, their attenuation by chemical and 

 physical means, or their primary neutralization by means of previ- 

 ously prepared antitoxins. In experimental work on animals the 

 first two methods are commonly employed and may be combined 

 with the third method. In man immunization by the use of toxins 

 is practised, mainly in connection with active immunization to diph- 

 theria. The combination between toxin and antitoxin is not in the 

 nature of a fixed and final reaction, and under certain circumstances 

 partial dissociation may occur. The active immunization of man 

 by the use of neutralized mixtures of toxin and antitoxin appears 

 to provide conditions whereby dissociation progresses gradually, 

 and the toxin is liberated in such small amounts that it does no 

 harm and yet induces in the body antitoxin formation. In the mean- 

 time the individual is protected by the antitoxin simultaneously 

 dissociated. Recent studies make it appear that several organisms 

 which formerly were supposed to produce only endotoxins elab- 

 orate in addition true toxins, and some of the earlier studies sup- 

 porting the assumption that antitoxins could be produced by these 



