Failure of Union between Antigen and Precipitin. 63 



these sera fixed complement. When "3" and "4" were mixed, 

 slow and slight, but distinct precipitates occurred, unmistakable 

 after 12 hours in the ice chest. 



On standing for one month in the ice chest sera "3" and "4" 

 were found to have spontaneously precipitated. The precipitate 

 showed slight complement fixation and the supernatant fluid was 

 found on titration to have lost about one half of its precipitating 

 power. 



Such spontaneous precipitation of precipitin sera has been 

 repeatedly observed after prolonged conservation. 



Von Dungern who has done much work on this question has 

 assumed a multiplicity of antigens and precipitins to account for 

 this failure of union between antibody and antigen in such sera. 

 He says "Neben dem gebildeten Praezipitin bleibt aber ein 

 anderer Theil der Praezipitablen Substanz der keine Affinitaet 

 zu dem gebildeten Praezipitin besitzt, bestehen, solange bis ein 

 anderes Partial Praezipitin von den Kaninchen geliefert wird, 

 welches sich mit Gruppen der in Loesung gebliebenen Eiweiss- 

 koerper vereinigen kann." 



We do not believe that this explanation is tenable in our cases 

 since eventual precipitation, therefore union, took place after 

 prolonged standing. 



Another explanation (Eisenberg) accounts for the phenomenon 

 by assuming that antigen and precipitin unite according to the 

 laws of mass action, establishing an equilibrium, in which un- 

 united portions of each of the reacting factors are therefore found. 

 This explanation seems incompatible with the observation that 

 the two sera did not bind complement, pointing to the absence of 

 united antigen and precipitin in the sera. 



We believe that the failure of union of the two elements may 

 be regarded as closely analogous to phenomena occurring under 

 the influence of protective colloids. 



This is compatible with the original failure of the two sub- 

 stances to unite; with their precipitation by other sera in which 

 there is present one or the other of the reacting bodies; and with 

 their final spontaneous precipitation on prolonged standing. 



It is more difficult to account, on this basis, for the mutual 

 precipitation of these sera which occurs when they are mixed. 



