294 APPLIED BIOLOGY 



that diphtheria toxins will cause the horse tissues to form 

 antitoxin, which appears in the blood, physicians now inject 

 into the blood of the human patient some antitoxin from 

 horse blood, and this saves the child from being seriously ill 

 while his own cells develop antitoxin. Thus it is possible to 

 put into the blood of a child on the first days of diphtheria 

 more antitoxin than the child might develop in his own cells 

 after days of illness. 



Some health laboratories keep inoculated horses constantly 

 so as to have a supply of antitoxin ready for use of doctors 

 who discover cases of diphtheria. The toxins injected into 

 the horses does not make them appear sick. The with- 

 drawing of comparatively small quantities of blood for 

 extracting the antitoxin is done by an instrument which does 

 no serious injury ; and more antitoxin may be obtained from 

 the same horse every month. 



264. Other Diseases are Similar. This story of the rela- 

 tion of bacteria to diphtheria is very similar to that of many 

 other diseases now known to be caused by bacteria. The 

 bacteria enter the body, multiply and form toxins. Then 

 antitoxins, or other opposing substances, appear, counteract 

 the toxins, and the patient recovers. In only a few cases 

 has it been possible to find in other animals antitoxins which 

 can be injected into the human tissues to cure or prevent 

 human diseases. 



Ant toxins and other antibacterial substances are specific. 

 For example, diphtheria antitoxin will not cure or prevent 

 any other disease, and the antitoxin which surgeons use after 

 injuries by Fourth-of-July pistols is obtained from animals 

 into whose bodies the toxins of tetanus or lockjaw bacteria 

 have been injected, causing the animal's tissues to make 

 tetanus antitoxin. 



266. Immunity. One of the most interesting things con- 

 nected with bacterial diseases is the fact that some people 

 never have certain diseases even when often exposed to infec- 



