DEFENSIVE FACTORS OF THE ANIMAL ORGANISM 195 



duced the virulence of anthrax cultures by growing them in the presence 

 of weak antiseptics (carbolic acid 1 : 600, potassium bichromate 1 : 5,000, 

 or sulphuric acid 1 : 200) . Cultivated under such conditions the bacilli 

 lost their ability to form spores and became entirely avirulent for sheep 

 within ten days. A similar result was later obtained by Behring ' when 

 attenuating B. diphtheria cultures by the addition of terchlorid of 

 iodin. 



Active Immunization with Sublethal Doses of Fully Virulent 

 Bacteria. — ^The use of fully virulent microorganisms in minute 

 quantities for purposes of immunization was first suggested by Chau- 

 veau,' and is naturally inapphcable to extremely virulent organisms 

 like B. anthracis. The principle, however, is perfectly valid, and has 

 been experimentally applied by many observers, notably by Ferran' 

 in the case of cholera. A similar method proved of practical value in 

 the hands of Theobald Smith and Kilborne * in prophylaxis against the 

 protozoan disease, Texas fever. 



Active Immunization with Dead Bacteria. — Suggested by Chau- 

 veau, the method of active immunization with gradually increasing doses 

 of dead microorganisms has been successfully employed by various ob- 

 servers, chief among whom are Pfeiffer, Brieger, Wright, and Wasser- 

 mann. The method is especially useful against that class of bacteria 

 in which the cell bodies (endotoxins) have been found to be incomparably 

 more poisonous than their extracellular products (toxins). From a 

 practical point of view, the method is of the greatest importance in 

 routine laboratory immunization against B. typhosus. Vibrio cholerse 

 asiaticse, B. pestis, and a number of other bacteria. In the therapy 

 of human disease, this method has recently come into great prominence, 

 chiefly through the work of Wright, whose investigations will be more 

 fully discussed in a subsequent section. 



Active Immunization with Bacterial Products. — Many bacteria 

 when grown in fluid media produce extracellular, soluble poisons which 

 remain in the medium after the microorganisms have been removed by 

 filtration or centrifugalization. Since the diseases caused by such 

 microorganisms are, to a large extent, due to the soluble poisons excreted 

 by them, animals can be actively immunized against this class of bac- 



> Behring, Zeit. f. Hyg., xii, 1892. 



' Chauveau, Compt. rend, de I'acad. des sci., 1881, t. xcii. 

 ' Ferran, Compt. rend, de I'acaxl. des sci., 1895, t. ci. 



* Th. Smith and Kitborne, U. S. Dept. of Agri., Bureau of Ani. Indust., Wash., 

 1893. 



