124 BACTERIOLOGY FOR NURSES 



but is shared, as we have seen, by certain substances in plants 

 (phytotoxins) and poisonous secretions of certain animals (zoo- 

 toxins). Red blood corpuscles, serum of different animals, egg 

 albumen, milk, etc., when inoculated into the tissues, are treated 

 as foreign substances and are disposed of as quickly as possible 

 by antibodies specially manufactured by the body cells for that 

 purpose. 



The term antibody is used to designate the entire group of spe- 

 cific substances produced by the body cells in reaction against 

 the various antigens; certain antibodies act by neutralizing their 

 antigen (antitoxin), others by agglutination or precipitation (agglu- 

 tinins or precipitins) ; others act by completely dissolving their 

 antigen (bacteriolysins, hemolysins) ; still others may so lower 

 the resistance of their antigen as to render it an easy prey to the 

 phagocytes (opsonins or bacteriotropins) . 



Mechanism of Immunity. Of the many theories advanced 

 regarding the mechanism of immunity two have claimed more at- 

 tention than all the others ; one, the " humoral theory," advanced 

 by Ehrlich, attributes the protective forces of the body to the body 

 fluids ; the other, the " cellular theory," proposed by Metchnikoff, 

 claims the honor for certain body cells. From recent studies, 

 however, it is evident that in different infections the body employs 

 different means of protecting itself. In certain diseases the pro- 

 duction of immunity seems to depend upon the activity of the 

 cells, in other diseases substances contained in the body fluids 

 seem to possess the property. 



Ehrlich's Side Chain Theory. The " humoral theory " of 

 immunity, which considers that the power of resistance to infection 

 resides in the body fluids, is said to have originated when Foder 

 discovered that the blood of a rabbit when placed in a test tube 

 will kill anthrax bacilli without the apparent aid of the body cells. 

 Later, in 1890, additional weight was given to the theory when von 

 Behring and Kitasato demonstrated the presence of antitoxin 

 in the blood. At first great importance was attached to the anti- 

 toxins, until it was discovered that they are produced only in a 

 few diseases, notably diphtheria, tetanus, and botulism. 



