602 IMMUNITY 



upon the development of a reaction-body or anti-substance, as 

 above described. We suppose that the serum antigens gradually 

 disappear from the body after the injection; from about the 

 eighth day onward anti-substances appear in the blood in large 

 amount, and if antigens are still present in proper amount, the 

 combination of the two, probably acted on by complement, 

 brings about the phenomena described. Manifestly, if the 

 antigens have disappeared before the anti-substances appear in 

 quantity, there will be no symptoms. At a later period anti- 

 substances will be present alone in the serum, and then the 

 injection of fresh antigens brings about an immediate reaction. 

 After the anti-substances have disappeared, the injection of 

 fresh serum causes no immediate reaction, but the mechanism 

 of reaction has been stimulated by the first injection; anti- 

 substances thus appear more quickly after the second injection, 

 hence the reaction is accelerated as compared with the reaction 

 after the first injection. 



