IONS IN IMMUNISATION 123 



proteins or bacteria under investigation are mixed 

 with the specific substance under consideration in 

 a salt-free condition no reaction occurs. The specific 

 substances may therefore be looked upon as giving 

 to the colloidal protein the properties of sensitive 

 colloids, and of being precipitated through the 

 presence of a small amount of salt-ions.* 



Again : two reacting colloids show an optimum 

 for precipitation, and an inhibition of the precipi- 

 tating process when one is in excess. This is a 

 generalised phenomenon of colloids in agglutination, 

 and, according to Biltz, is observed in nearly all 

 immune-body reactions. The bacterial antitoxin 

 sera show a maximum effect when they contain a 

 medium amount of the immunising substance. 



The proteins, including the antibodies, according 

 to Landsteiner, represent amphoteric substances ; 

 that is, substances which assume basic properties in 

 acid substances and acid properties in alkaline solu- 

 tions ; they change the sign of their electrical charge 

 with a change in their reaction. " But there exists 

 a zone between the extreme changes of their elec- 

 trical charges when these hermaphrodite-like sub- 

 stances respond to the slightest change in their sur- 

 roundings." That these normally sensitive colloids 

 have at one time a positive and at another time a 

 negative kind of electricity is evident from facts ; 

 and Landsteiner's attempt to explain the specificity 

 of antibodies is based upon his conception of the 

 role played by these amphoteric substances. 



The side-chain theory of Ehrlich, developed by 



* Pauli's " Physical Chemistry in the Service of Medicine," 

 p. 123. 



