354 COLLECTED STUDIES IX IMMUNITY. 



Thus complement-diverting groups pre-existing in normal serum of 

 the species in question, and which have not, therefore, originated 

 through immunization, may be present or may be set free by the 

 inactivation (normal anticomplements, etc.). The question which 

 arises, namely, whether one is dealing with a deflecting body of nor- 

 mal serum or with one produced by immunization, can, of course, 

 be decided by the previous investigation of the normal serum of the 

 animal in question, as well as by comparison with several other nor- 

 mal sera of the same species 



In all of these cases, however, the plates with the largest amounts 

 of immune serum will show the least bactericidal action, i.e., the 

 largest number of colonies. From this it follows that one can err 

 in judging the bactericidal power of a serum if only larger amounts 

 of immune serum are used for the bactericidal test (about 1.0, 0.3). 

 Thus in the beginning we overlooked the high bactericidal power 

 of a dysentery serum (Shiga), for this became manifest only after 

 we employed doses of 0.025 immune serum and still less. 



The deflection of complement just mentioned, by means of ambo- 

 ceptors produced by immunization (or by amboceptoids), permits 

 of another method of testing by which also the serum can be shown 

 to be a specific immune serum. For this purpose one uses an active 

 normal serum bactericidal in itself or a mixture of inactive immune 

 serum and a complement. By means of a preliminary test one 

 determines the amount of serum or serum mixture which completely 

 kills the amount of culture sown. To such a dose of serum or 

 serum mixture (bactericidal in itself) decreasing amounts of in- 

 active immune serum are added, when it will usually be found that 

 the phenomenon of deflection of complement again appears. This 

 manifests itself by the fact that the plates with the larger amounts 

 of immune serum show a larger number of colonies, the number of 

 these decreasing in proportion with the amount of immune serum 

 added. 



In order to interpret the results of the plate tests correctly it 

 is first necessary to be sure whether one is dealing with a normally 

 pre-existing deflecting body or with one produced by immunization 

 (see above). By means of combining experiments it must also 

 be shown whether the deflection is caused by amboceptors or ambo- 

 ceptoids. It is not difficult, by binding them to the corresponding 

 bacteria, to remove the amboceptors produced by immunization. 

 In most cases the addition of a moderate amount of bacteria care- 



