CHANGES WROUGHT IN PATHOLOGY. 123 



The phenomena observed in precipitin and agglu- 

 tinin reactions are explained in a similar way. These 

 precipitations are possible only in the presence of salts. 

 If the protein or the bacteria under investigation are 

 mixed with the specific substances in a salt-free condition, 

 no reaction occurs. These specific substances may, 

 therefore, be looked upon as giving the colloidal proteins 

 the properties of sensitive colloids, that of being pre- 

 cipitated through small amounts of salt ions. According 

 to BILLITZER the specific substances serve in this case 

 only to give the colloidal particles the charge and size 

 necessary for precipitation. Apparently all " sensitizing " 

 reactions encountered in the realm of the immune-body 

 reactions are explainable in a similar way. 



A phenomenon frequently observed is furnished by the 

 above-mentioned tendency of colloids to show an opti- 

 mum proportion in which the two reacting colloids must 

 be mixed in order that they may be precipitated, and by 

 the inhibition of the reaction when an excess of the one is 

 present. BILTZ has already pointed out the existence of 

 this generalized phenomenon of colloids in agglutination; 

 analogous phenomena may, however, be observed in 

 nearly all immune- body reactions. A remarkable exam- 

 ple of this kind is furnished by the " complement diver- 

 sion " (Komplementablenkung) observed by NEISSER and 

 WECHSBERG. These authors showed that bactericidal 

 immune sera showed a maximum effect, under other- 

 wise similar conditions, when they contained a medium 

 amount of immune substance. No doubt we can with' 

 profit now express the description of this phenomenon 

 in the smoother language of colloid chemistry. The com- 

 plex relations existing in the case of haemolysis, which 



