114 IMMUNE SERA 



A very important fact in connection with the pro- 

 duction and identification of antibodies is, that the 

 combination of antigen with its antibody is specific, 

 that is, the antibodies will combine with the anti- 

 gens which induced their formation and with no 

 other. For example, if a serum is thought to con- 

 tain gonococcus antibodies, some gonococcus anti- 

 gen is added to it in a test-tube. If combination 

 takes place it is evident that the serum contained 

 gonococcus antibodies, for if it did not there would 

 be no combination, although antibodies for other 

 organisms might be present in abundance. This 

 antigen-antibody combination is able to absorb 

 a third substance; this third substance is found in 

 the blood serum of all normal animals and because 

 it completes the above combination it is called 

 complement. Neither the antigen alone, nor the 

 anti-serum alone, has the slightest effect on com- 

 plement. Moreover, complement is not absorbed 

 when an anti-serum is placed in contact with an 

 antigen other than the one which caused its pro- 

 duction. 



The affinity of an antigen-antibody combination 

 for complement is not only specific but is also quan- 

 titative and the greater the amount of antibodies 

 present in the serum the greater will be the amount 

 of complement absorbed. Using the gonococcus 

 antigen as an example the above facts may be illus- 

 trated as follows : 



