68 INFECTION AND IMMUNITY 



various substances favourable or unfavourable to the 

 development of particular disease germs may, in cer- 

 tain cases, be the fundamental cause of race immun- 

 ity. It has been shown by experiment that natural 

 immunity may be overcome in certain animals by 

 inoculating them with disease germs mixed with 

 certain chemical substances or with sterilised cult- 

 ures of various bacteria. The susceptibility of the 

 victims of chronic alcoholism to infection by various 

 pathogenic bacteria is well known. Whether this is 

 due to the presence of alcohol or to chemical changes 

 in the body fluids resulting from its use is not de- 

 termined. A complete knowledge of the facts would 

 probably show that immunity, natural or acquired, in 

 the ultimate analysis, to a large extent has a chemical 

 basis that is, it depends upon the presence of some 

 substance which exercises a deleterious influence 

 upon the germ or neutralises its toxic products. 



That the blood-serum of healthy animals contains 

 substances which have a decided germicidal effect 

 has been demonstrated by experiments made with 

 blood withdrawn from the circulation. This property 

 belongs to the clear serum which is obtained after 

 coagulation of the fibrin and separation of the clot 

 containing the red and white blood corpuscles. When 

 the blood-serum is kept for some time it loses its 

 germicidal activity. This is also destroyed by heat, 



