APPENDIX. 



149 



As can be seen from Ehrlich's diagrams, the 

 bacteria or blood cells combine directly only with 

 the immune body. The complement, as already 

 said, has no way of laying hold of the cells. As 

 soon as the bacteria or cells have anchored the 

 immune body, however, conditions change. The 

 combination at once attracts and unites with the 

 complement. If the amount of complement is not 

 too large, the combination may unite with all of it, 

 i.e. may abstract the complement from the serum. 



Just let us examine this by means of an illustra- 

 tion : Let us suppose we have immunized an ani- 

 mal with typhoid bacilli, and have obtained a 

 specific serum directed against these bacilli. This 

 serum has been inactivated by heating it to 55 C., 

 so that now it will act on typhoid bacilli only when 

 some fresh normal serum is added to complement 

 the immune body. For this purpose we have 

 provided ourselves with some freshly drawn serum 

 from a guinea pig. The guinea pig serum, there- 

 fore, is the " complement." On mixing typhoid 

 bacilli with the specific immune serum and then 

 with the complement, these three factors enter into 

 combination, and this results in the destruction of 

 the typhoid bacilli. The quantities can easily be 

 so arranged that this combination uses up all of the 

 complement, so that the fluid contains not a trace 

 of free complement after the substances have com- 

 bined. 



Suppose, now, that we also had a specific serum 

 obtained by injecting an animal with red blood 



