16 VACCINE AND SEKUM THERAPY 



of 56° C. By adding to this inactivated serum normal 

 fresh serum, it again becomes reactivated. From 

 this observation Bordet drew the conclusion that the 

 bactericidal action of the serum depended on two 

 distinct substances. The one present in normal 

 serum and thermolabile he conceived to be identical 

 with Buchner's alexin; the other, more stable, pro- 

 duced, or at least increased, in the serum by the 

 process of immunisation, he called the " sensitising 

 substance." This substance, he believed, acting upon 

 the bacterial cells, rendered them vulnerable to the 

 action of the alexin. Without the previous prepara- 

 tory action of the " sensitising substance " the alexin 

 was unable to act, and likewise without the co-operation 

 of the alexin the "sensitising substance" produced 

 no visible effects. The same laws govern the pheno- 

 menon known as hemolysis as that of bacteriolysis. 



Since the thermolabile substance or alexin or com- 

 plement — more widely known by the last name — was 

 always present in normal serum and had been shown 

 to be little, if at all, increased during immunisation, 

 this substance could have but little relation to the 

 changes taking place in the animal body as immunity 

 was acquired. The more stable '* immune body " was 

 the one which seemed specifically called forth by the 

 process of active immunisation. According to Ehr- 

 lich's theory, then, it amounts to this, that when 

 bacteria or blood cells are injected into the body of 

 an animal certain atom groups or chemical com- 

 ponents of the injected substance were united to 



