HISTORIC DEVELOPMENT OF IMMUNOLOGY 41 



acted upon the immunity is said to be anti- 

 toxic. 



ANTIBODIES 



Two explanations have been advanced to ac- 

 count for acquired immunity : the humoral theo- 

 ry attributes it to soluble substances in the 

 blood-serum; while, according to the cellular 

 theory, it depends on the activity of phagocytic 

 cells. Whatever may be the essential protective 

 feature, we can demonstrate that when an ani- 

 mal becomes immune its blood-serum acquires 

 new properties which we attribute to the pres- 

 ence of specific soluble substances called im- 

 mune bodies or antibodies. 



Any substance which, when introduced into 

 the body can stimulate the production of such 

 antibodies is called an antigen. Most antigens 

 are protein substances, and many proteins that 

 are foreign to an animal exercise antigenic pow- 

 ers if introduced into its blood or tissues. It 

 may be an infectious or harmless bacterium, an 

 animal cell, or a toxic or an innocuous protein. 

 Toxins and enzymes of unknown chemical com- 

 position, certain glucosides and possibly certain 

 lipoids may also act as antigens. 



We infer the existence of antibodies entirely 



