40 MODERN BIOLOGIC THERAPEUSIS 



mentally produced. Experimental, artificial or 

 intentional active immunization is usually called 

 "vaccination", and generally produces in the 

 individual a mild form of the symptoms usually 

 found in the infection. Active immunization, 

 protective and curative, is most frequently at- 

 tempted with injections of killed bacteria; al- 

 though toxins and living organisms as well as 

 attenuated viruses are used to immunize against 

 certain infectious diseases. Attenuated or killed 

 organisms prepared for immunization are re- 

 ferred to as "vaccines." 



Passive Immunity Acquired passive immu- 

 nity takes place as the result of the introduction 

 of immunizing substances that have been pre- 

 pared by actively immunized individuals or ani- 

 mals. This is usually conferred by the injec- 

 tion of blood-serum from immunized animals. 

 There are two classes of immunizing sub- 

 stances : those acting on bacteria are said to be 

 antibacterial; while those acting on toxins are 

 called antitoxic. If the resistance depends on 

 the ability to destroy the invading parasite, it 

 is called antibacterial immunity; whereas, if it 

 depends on the ability to neutralize the toxin of 

 the parasite the bacterial body itself not being 



