34 MODERN BIOLOGIC THERAPEUSIS 



invading bacteria and the leukocytes. Von Beh- 

 ring's discovery in 1890 that antitoxin is a solu- 

 ble substance present in blood-serum showed the 

 inadequacy of this explanation, and it was soon 

 found that immune serum possessed other spe- 

 cific properties. In 1893 Denys and Leclef 

 showed that even the increased activity of the 

 leukocytes of an immune animal was due to sub- 

 stances in the blood-serum and not to any 

 changes in the leukocytes themselves. Bordet 

 added to the knowledge of bacteriolysis by 

 showing that two distinct substances ambo- 

 ceptor and complement take part in the solu- 

 tion of bacteria, either being inactive of itself. 

 Side-Chain Theory The most important at- 

 tempt to explain the phenomena of immunity 

 was the so-called ' ' side-chain theory ' ' developed 

 by Ehrlich in 1897. He believed that these phe- 

 nomena were essentially chemical reactions, and 

 applied to immunity certain ideas which he had 

 developed to account for the combination of 

 food stuffs with tissue-cells. Ehrlich conceives 

 the individual cell to be a complex molecule, 

 comprising a primary central nucleus to which 

 are attached its secondary atom-groups, side- 

 chains, or receptors. Their principal function 



