212 ANTI-BACTERIAL PROPERTIES OF SERUM 



of the serum with heterologous organisms, the bactericidal 

 action can be more than restored to its former value by the 

 addition of a small quantity of the homologous immune- 

 body (artificial anti-serum). 



5. Normal bactericidal action may in certain cases differ 

 from bactericidal action through the medium of an artificial 

 immune-body ; in the former it may be due to the union 

 (direct or indirect) of a few very active molecules of comple- 

 ment with the bacterium, in the latter to union of a larger 

 number of more weakly acting complement molecules. 



6. The fact that the bactericidal effect for an organism 

 is usually most readily reduced by treatment with the same 

 organism may be due to the existence of natural immune- 

 bodies ; it may also be due, however, to variations amongst 

 the molecules of bacteriophilic complement. This question 

 is left undetermined. 



ADDENDUM. ON COMPLEMENTS IN GENERAL 



The complement content of the serum is of prime impor- 

 tance in immunity, and no doubt also in the normal nutritive 

 processes of the body. It includes a number of molecules 

 of different kinds, or probably more correctly the molecules 

 show an infinite number of gradations in their affinities and 

 action, and probably also vary from time to time. We 

 have, of course, no direct chemical method of demonstrating 

 the existence of complement molecules. This can only be 

 done by their effect in producing haemolysis, bacteriolysis, 

 &c. Complement may, however, enter into combination 

 with various organic substances without producing any 

 recognizable effect, the occurrence of the combination being 

 shown by the absorption or subtraction method, which 

 has so often been used in the foregoing investigations. This 

 method must accordingly be employed where questions of 

 combination are concerned. In the present state of our 

 knowledge we cannot define complement according to the 

 action which it produces, as there may be molecules of the 



