Fixation Reaction to Specific Precipitation. 139 
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The relation of the fixation reaction to specific precipitation. 
By FREDERICK P. GAY. 
[University of California.] 
This work represents a continuation of studies on the mechan- 
ism of the fixation reaction between a serum and its antiserum 
begun by the author in 1905. It was shown that this fixation 
which had been attributed by Gengou to the presence of anti- 
albuminous sensitizers was apparently produced by the precipitate 
formed in the serum-anti-serum mixture. A prolific literature has 
since sprung up engaged principally in establishing the presence 
or absence of parallelism between fixation and precipitation. The 
results in either direction are far from conclusive. 
In the present work an attempt has been made to settle the 
question by studying more attentively an instance in which both 
precipitation and fixation are known to occur, and where one may 
reasonably be associated with the other. It is found that in 
addition to the voluminous precipitate which is known to be 
formed by mixture of an excess of the antiserum with the anti- 
genic serum, there is another precipitate produced with certain 
individual antisera in the presence of a large excess of antigen. 
This latter, and we believe newly recognized, zone of precipitation 
lies above the zone of inhibition produced, as is well known, by 
the ordinary excess of antigen. The upper zone precipitate 
differs in its granular appearance and slowness of formation from 
the lower zone precipitate. 
No fixation of alexin occurs as a rule in mixtures representing 
the inhibition zone and never in presence of the upper-zone 
precipitate which may, however, equal or exceed in volume a 
precipitate of the lower zone which produces complete fixation. 
This fixation in the lower zone precipitate mixture is produced 
almost invariably by the washed precipitate and not by the super- 
natant fluid. The latter fluid may, however, in some cases give 
partial fixation. 
It is well recognized that the addition of a small amount of 
antigen to the formed precipitate will dissolve it. This applies 
