86 ELEMENTS OF BIOLOGY 



lation. But the presence of the small amount o£ toxin from this local 

 attenuated culture is sufficient to set in action the defense mechan- 

 ism and the body cells pour out antibodies. Thus an immunity is 

 conferred that usually persists for a long time. In the case of 

 typhoid fever and certain other diseases, it has been found that the 

 dead bodies of the bacteria, when broken up, release a substance 

 that when injected into the human will cause the body to produce 

 typhoid antibodies. So cultures of typhoid bacteria are killed and 

 used as inoculations to bring about immunity. Attenuated cultures, 

 such as smallpox described above, and killed cultures, such as ty- 

 phoid, when used to produce artificial immunity are called 



VACCINES. 



Serums. The natural immunity mechanism of lower animals 

 to certain diseases which aflfect man is taken advantage of in the 

 treatment and prevention of human disease. For example, in the 

 blood stream of the horse inoculated with diphtheria is to be found 

 a relatively high concentration of diphtheria antitoxin, sufficient to 

 prevent the development of the disease in that animal. Thus the 

 horse so inoculated with diphtheria is a convenient source of anti- 

 toxin and the blood of that animal is withdrawn in small quan- 

 tities, the blood cells removed and the resulting serum rectified and 

 standardized and used by injection to increase the antitoxin in the 

 blood of a human affected with diphtheria. The powers of the 

 human body to produce antitoxin are thus supported by added 

 antitoxin from the horse and the amount at hand for defense 

 against the invasion of diphtheria bacteria materially increased. This 

 method of treatment is known as serum therapy. The search for 

 serums rich in antibodies is continuous and from time to time new 

 successes are attained. 



Sterilization. Disease-producing bacteria may be killed in a 

 number of ways. Most chemicals when present in sufficient strength 

 will destroy them. However, it is not practical to introduce strong 

 chemicals into the human bodv, for in concentrations that kill 



