224 GENERAL BIOLOGY OF MICRO-ORGANISMS 



animals to immunize them, especially in the case of diphtheria 

 and tetanus, the bacteria of which produce very powerful soluble 

 toxins. As a result of this treatment antitoxins are produced and 

 circulate in the blood of the animal. 



Bacterial extracts, either those contained in inflammatory 

 exudates, the so-called aggressins of Bail, or extracts obtained by 

 soaking bacteria in blood serum or in distilled water, the so-called 

 artificial aggressins of Wassermann and Citron, have proved of 

 value in experimental immunization of animals against many dif- 

 ferent bacteria. It is claimed that the reactions to injection are 

 exceptionally mild, while the resulting immunity is more solid. 

 Certain products of the disintegration of typhoid bacilli have been 

 obtained by Vaughan, which possess considerable immunizing 

 power, but apparently only slight toxicity. None of these bac- 

 terial extracts has yet passed beyond the experimental stage in the 

 immunization of man against a disease. 



A certain slight grade of immunity may be secured in some 

 instances by procedures which seem to bear no relation to the 

 specific micro-organisms in question. Thus the injection of cul- 

 tures of B. prodigiosus and B. pyocyaneus results in an increased 

 resistance to infection with anthrax. Similar increased resistance 

 has been observed to follow a simple surgical procedure, such as 

 section of the sciatic nerve. The explanation of these results is 

 not clear, but perhaps the effect may be attributed to a general 

 stimulation of the body defenses, especially the phagocytes. 



Passive Immunity. Passive immunity is produced by inject- 

 ing into the body a fluid taken from another animal which contains 

 antitoxins, bacteriolysins, opsonins or other substances known 

 as immune bodies. The animal which furnishes the immune 

 bodies must be first actively immunized, and it possesses an ac- 

 tive immunity. If its blood plasma be drawn and injected into 

 a child, the child acquires a borrowed immunity without the 

 necessity of any active participation of its own cells in the pro- 

 duction of immune bodies. The possibility of producing such 

 passive immunity has been demonstrated in a number of diseases. 



