IMMUNITY 59 



It is supposed that in active immunity, i.e. when due 

 to virulent organisms, the body-cells continue to 

 maintain the antitoxin above the normal; while in 

 passive immunity, i.e. when due to antitoxin serum 

 taken from an immunized animal, that the antitoxin 

 is eliminated from the body after a certain time, 

 and the body-cells are not stimulated to continued 

 activity. This condition is notable following the 

 administration of diphtheria antitoxin, immunity 

 lasting about three weeks. 



It might be supposed the presence of antitoxin, 

 or the abiUty of the body-cells to produce it, would 

 be suflBcient defense against disease-producing bac- 

 teria, but there seems to be at times what is known 

 as a chemical affinity (chemiotaxis) between the 

 toxins and the tissues of the body, in which the 

 toxins combine with the body-cells instead of with 

 the antitoxin, which causes the death of the cells. 



Bactericidal (ability to destroy) Substances. — The 

 blood can not only neutralize bacterial toxins, but 

 can destroy the bacteria; it is said that a single drop 

 of rabbit blood may destroy over fifty thousand 

 anthrax bacilli. 



The natural immunity of animal organisms was 

 thought by Buchner and others to be due to the 

 bactericidal substances in the blood, called by them 



