14 VACCINE AND SERUM THERAPY. 



THEORIES OF IMMUNITY. 



Acquired immunity, as has been stated, results because some 

 individual or animal has gone through a natural or modified 

 course of the disease. While acquiring immunity, the body and 

 its tissues have in some way been modified. Various theories 

 have been advanced to explain this phenomenon. 



Klebs and Pasteur tried to explain the changes that occur 

 in the acquisition of immunity by assuming that in going through 

 a natural or modified course of disease, certain substances, nec^ 

 essary as food for the parasites, are used up. As the result of 

 immunization, the food necessary 7 for the microorganism is con- 

 sumed and the individual is immune to a certain organism be- 

 cause this organism cannot obtain the food it needs for its ex- 

 istence and production of the disease. 



Chauveau assumes that in immunization certain products 

 of bacterial metabolism are retained in the body of the immun- 

 ized animal or individual, which products protect the body tissues 

 from further invasions by that particular parasite. 



MetchnikofT, in 1883, formulated a theory according to which, 

 during immunization, certain white blood cells and cells of certain 

 organs acquire the ability to engulf and destroy the attenuated 

 bacteria.. This results in acquiring the ability to engulf and de- 

 stroy more virulent forms of this same species of microorganisms. 



EHRLICH'S SIDE CHAIN THEORY OF IMMUNITY. 



In 1887 Salmon and Smith, Foa and Bonome, Roux and 

 Chamberland, and others found that immunity could be produced, 

 not only bythe injection of bacteria, but also, as a result of the injec- 

 tion of the products of bacterial metabolism. As a result, a chemi- 

 cal theory, of immunity was advanced. According t<? this theory 

 the tissues of the body are chemically changed by immunization. 



Fodor, in 1887, was the first to observe that the normal body 

 fluids, especially the blood, contain certain substances able to 

 destroy bacteria. Buchner, Behring, and Nuttall, soon after 

 Fodor's observation also recognized the bactericidal powers of 

 certain sera. In 1889 Buchner reported that the cell -free blood 

 serum contains certain substances which he called alexines. 

 Alexines, he found, have the property, of destroying bacteria. 



