574 BACTERIOLOGY. 



tain that they exhibit their protective functions less by 

 direct combination with the toxins than by a specific 

 stimulation of the tissue-cells that enables the latter to 

 resist the harmful influences of the poisonous bacterial 

 products. On the other hand, Behring, 1 Ehrlich, 2 and 

 their associates contend that they are vital tissue ele- 

 ments, having the property of combining directly with 

 the toxins to form " physiologically neutral " toxin-anti- 

 toxin compounds that are in a manner analogous to the 

 double salts of certain chemical reactions. 



NATURAL, IMMUNITY. It is well known that among 

 man and the lower animals individuals are frequently 

 encountered who are, in general, less susceptible to infec- 

 tion than are others of their species ; and that particu- 

 lar species of animals not only do not suffer naturally 

 from certain specific diseases, but resist all efforts to 

 produce the diseases in them by artificial methods ; in 

 other words, they are naturally immune from them. 

 The term " natural immunity," as now employed, 

 implies a congenital condition of the individual or 

 species, a condition peculiar to his idioplasm, which has 

 been transmitted to him as a tissue-characteristic through 

 generations of progenitors. 



ACQUIRED IMMUNITY. Again, it is often observed 

 that an individual or an animal after having recovered 

 from certain forms of infection has thereby acquired 

 protection from subsequent attacks of like character; in 

 other words, they are said to have acquired immunity 

 from this disease. "Acquired immunity" implies, 

 therefore, a condition of the tissues of an individual, 



1 Behring : Infektion und Desinfektion, Leipzig, 1894, S. 248. 



2 Ehrlich : Klinisches Jahrbuch, 1897, Bd. vi. Heft 2, S. 311. Fort- 

 schritte der Medicin, 1897, Bd. xv. No. 2, 



