228 GENERAL BIOLOGY OF MICRO-ORGANISMS 



results in destruction of the side-chains attacked, and in regen- 

 erating these the cell over-compensates, the excess side-chains, 

 receptors of the first order (see page 209), being set free into the 

 blood and constituting the antitoxin, which is capable of neutral- 

 izing l toxin there or in the test-tube. The assumption of two 

 chemical groups in the toxin molecule is strengthened by the 

 observation that diphtheria toxin changes on standing so that 

 its poisonous property is much diminished without corresponding 

 loss of ability to combine with antitoxin. Such changed toxin, 

 in which the haptophorous group persists while the toxophorous 

 group has degenerated, is called toxoid. In order to explain the 

 formation and structure of agglutinins and precipitins, Ehrlich 

 assigned a more complex composition to the side-chains which 

 constitute these substances, leading to the conception of a receptor 

 of the second order (see page 210), with its haptophorous and 

 zymophorous groups. In the case of the cytolysins, a further 

 amplification of the idea was necessary to explain the observed fact 

 that the cytolysis is due to two components, one of which is a 

 thermolabile, normal constituent of the blood and not increased 

 as a result of immunization, the other being a thermostable sub- 

 stance which is produced as a result of the immunization process. 

 This latter immune body, the receptor of the third order, was there- 

 fore pictured as a double receptor (amboceptor) capable of attach- 

 ing on the one hand the foreign body (antigen) and on the other 

 the normal component necessary to complete the lytic complex, 

 to which component .the name complement was given. 



With the recognition of opsonins by A. E. Wright in 1903, 

 the opposing theories of the French and the German schools be- 

 gan to be reconciled, and the relatively simple and largely hypo- 

 thetical theories of ^jfnmunity be^ga'n to give way to a more exact 

 and necessarily' more complex science df immunology. Bordet 

 and his -pupils Reserve credit for leading the reaction against too 

 slavish adherence to theory in the study of immunity. Our 

 modern ideas are no longer confined within the scope of any one 

 theory and it is necessary to recognize the existence of a great 



