ANTI-INFECTIOUS SERUM. 485 



it will not protect against the injection of the soluble 

 toxin. 



The antitoxic serum can be used as a preventive, or as~ 

 a curative agent whereas the anti-infectious serum has a 

 preventive value only. Even this action is manifest only 

 under certain experimental conditions. Thus, the anti-infec- 

 tious cholera serum, which protects perfectly against sub 

 cutaneous or intraperitoneal injections of the cholera 

 vibrio, is of no value if the organism is introduced into the 

 intestinal canal. The soluble poison elaborated by the 

 vibrio in the intestines is absorbed and can be counter- 

 acted or rendered inert only by an antitoxic serum. 



An animal that has recovered from an attack of a dis- 

 ease or has been rendered immune by treatment with a 

 germ or a toxin is said to have acquired active immunity. 

 The blood of an actively immunized animal is capable, even 

 in a small dose, of conferring a temporary exemption from 

 that disease. This is designated as passive immunity. 

 Thus, the . blood of an animal actively immunized against 

 the soluble diphtheria toxin will, if injected, confer passive 

 immunity to man or animals. In other words, the soluble 

 toxin or the virulent germ induces active immunity, where- 

 as the antitoxic, or anti-infectious serum obtained from 

 such actively immunized animal induces a condition of 

 passive immunity. 



The organism employed for immunizing- purposes should possess 

 a maximum degree of virulence. This can be obtained by repeated 

 passage from animal to animal (p. 278), or by cultivation in collodium 

 sacs (p. 496), alternating with passage through animals. The cultures 

 should be grown on agar and should not be more than 20 or 24 hours 

 old. 



When it is desired to obtain a large surface growth of bacteria 

 the Roux flask shown in Fig. 69 will be found extremely useful. 100 

 c.c. of agar or more are placed in the flask and sterilized. On placing 

 the flask in a horizontal position the agar solidifies and a large sur- 

 face is thus obtained. It can be inoculated by introducing a few 

 drops of the culture suspended in bouillon, or better by swabbing the 



