i8o THE PRINCIPLES OF IMMUNOLOGY 



be supposed that the antigenic protein can simultaneously combine with 

 precipitins and with amboceptor. He offers the hypothesis that one 

 immune molecule may contain different binding complexes, one, for 

 example, combining with precipitins to produce a precipitate and the 

 other combining with the antigen and the complement to produce fixa- 

 tion. If this view be accepted, the experiment of Friedberger and Lief- 

 mann, in which the immune serum was heated, indicates that of the 

 two binding complexes the precipitating one is the more labile. It can 

 very readily be seen that the interpretation of Sachs depends almost 

 entirely upon an acceptance of the Ehrlich hypothesis of the structure 

 of immune bodies. 



Dean has examined the question and finds that the optimal rela- 

 tionship between antigen and immune serum for the production of 

 precipitation is by no means necessarily the optimal relationship for 

 complement fixation. Therefore, the two phenomena, as has already 

 been pointed out, are by no means parallel. He is of the opinion, 

 however, that this lack of parallelism is not necessarily an indication 

 that the two things are entirely distinct and separate. He is of the 

 opinion " that they represent two phases of the .same reaction." The 

 complement fixation represents the earliest and more delicate stage 

 of a reaction which, in its more marked manifestation, is seen by the for- 

 mation of a precipitate. Zinsser has studied the matter carefully and has 

 come to the conclusion " that the precipitation is merely a secondary, 

 colloidal phenomenon, which may, or may not, coincide with the phase 

 of greatest alexin (complement) fixation, according to other fortuitous 

 conditions which may favor or retard flocculation." He found that a 

 mixture of sheep serum and its specific immune serum showed com- 

 plement-fixing activity only in the precipitate. On the other hand, in 

 a mixture of a filtrate of typhoid bacilli and a specific immune serum 

 both the precipitate and the supernatant fluid were capable of fixing 

 complement. " From this it seems to follow that immunization with 

 the more complex cellular elements has given rise to the precipitating 

 antibodies present also in the anti-sheep serum, and in addition to 

 this to sensitizers which are not precipitable (remaining in the super- 

 natant liquid) and not present in the anti-sheep serum." He, therefore, 

 is of the opinion that since both the antigen and the immune body are 

 colloidal in character they may be expected to follow the laws of 

 colloids. This may be interpreted to indicate that the contact of the 

 mutually precipitating colloids must be present in optimal concentration 

 in order to show a visible precipitation, but, on the other hand, the 

 interaction of the two bodies which, in the quantities employed, 

 show no visible precipitate, may be demonstrated by the comple- 

 ment-fixation test. He states " that the visible precipitation would 

 seem, therefore, to be a secondary phenomenon, the essential one 

 being- the union of an antigen with a sensitizer by which it is ren- 

 dered amenable to the action of the alexin " (complement). 



Is the Complement-fixing Body an Amboceptor? There arises 

 further the question as to whether or not the body, which, in combina- 



