IMMUNITY AND SUSCEPTIBILITY. 123 



vidual's life. In other cases it is only temporary, rarely 

 continuing longer in diphtheria than thirty or sixty 

 days. Passive immunity is probably never permanent, but 

 continues only as long as the antitoxin is uneliminated. 

 In cases in which diphtheria antitoxin is used for pro- 

 phylactic purposes it seems to be effective for two or 

 three months. Forced immunity is not permanent, but 

 begins to decline as soon as the treatment of the animal 

 is suspended, the return to the normal condition being 

 slower than the production of the abnormal one. 



Two very important phenomenal manifestations of the 

 blood serum of animals with forced immunity must be 

 mentioned, namely, the antitoxic and the antimicrobic 

 poivers. 



The Antitoxins. — By antitoxin is meant a peculiar 

 protective energy manifested by the blood serum and other 

 fluids of animals subjected to a high degree of forced im- 

 munity. 



The first observation upon the protective power of 

 immune blood was probably made in 1890 by Ogata and 

 Jasuhara, 1 who found that when animals were given a 

 subcutaneous injection of blood from an animal im- 

 munized to anthrax, they were able to resist the effects 

 of inoculation with a virulent culture. In the same year 

 Behring and Kitasato 2 found that the blood serum of an 

 animal immunized to diphtheria or tetanus when added 

 to a culture of the respective bacilli neutralized its 

 power to provoke disease, and that when added to the 

 filtered culture it had the power of destroying its toxic 

 effects. 



The next year, 1891, Kitasato 3 discovered that when 

 mice were inoculated with tetanus and symptoms of the 

 disease appeared, they could be saved by the intra- 

 abdominal injection of blood serum of an immunized 

 mouse. About the same time Ehrlich * immunized ani- 



1 Loc. cit. ' Deutsche med. Wochenscrift, 1890, No. 49. 



■ Zeitschrift fiir Hygiene, 1892, xii., p. 256. 



* Deutsche med. IVochenschrift, 1891, Nos. 32 and 44. 



