MODE OF ACTION OF BACTERICIDAL SERA. 129 



bacteria, and a bacterium all of whose receptors are laden with 

 b need not at all be injured in its vitality. Body b normally possesses 

 a peculiar function, namely, to serve as a coupling member or link, 

 and hence it possesses two groups (amboceptor). In this particular 

 case one of these groups fits the receptor of the bacterium, the other 

 possesses a peculiar relation to those normal ferment-like constitu- 

 ents of sera which Ehrlich has termed complements. When there- 

 fore to a normal serum which contains suitable complement we 

 add equivalent amounts of immune serum, the condition pictured 

 in A I will result. On adding the corresponding bacterium to this 

 we get the condition shown in A II, in which all the bacterial receptors 

 are occupied with immune bodies, or, more accurately, with immune 

 bodies which on their part are loaded with bacteriolytic comple- 

 ment c. In the case here presented we shall say that it requires 

 the occupation of all the receptors with complemented interbodies 

 to cause the death of the bacterium. 



If now to an equivalent mixture of complement and interbody 

 we add an excess of interbody, it will be possible for only a part 

 of the interbody to be loaded with complement, leaving a portion 

 of the interbody uncomplemented. On adding the corresponding 

 bacteria a number of conditions may result ; the affinity of the inter- 

 body for the bacterial receptor may, as a result of the loading with 

 complement, (1) remain unchanged, (2) it may thereby be increased, 

 or (3) be diminished. 



In the figure, B II shows the condition of increased affinity. 

 Of the six interbodies only those combine with the bacterium which 

 have become laden with complement. In this case therefore the 

 excess of interbodies will have no influence on the bactericidal effect. 

 The condition is really the same as A II, except that free interbody 

 is also present. 



C II shows the condition of unchanged affinity. In this case, 

 if we add the bacterium to the mixture of complement and excess 

 of interbody, all the receptors of the bacterium will, to be sure, be 

 occupied by interbodies, but this will be entirely without any regard 

 to the fact that these interbodies are or are not loaded with com- 

 plement. It may therefore happen that only a few of the bacterial 

 receptors will be occupied by complemented (i.e. active) interbodies, 

 while the rest of the bacterial receptors are occupied by uncom- 

 plemented (hence inactive) interbodies. As already mentioned, 

 however, the vitality of such a bacterium is not necessarily destroyed. 



