CHAPTER 



EVOLUTION AS SEEN IN 

 SEROLOGICAL TESTS 



The Precipitin Test 



Serological tests depend upon the faculty possessed by 

 the body of protecting itself against foreign invaders. When bacteria and 

 viruses gain entrance into the body the latter responds with a defense 

 mechanism consisting of the formation of substances called antibodies. 

 When, for example, a person has smallpox his body forms antibodies 

 against smallpox virus. The antibodies react with the virus, aiding the 

 body to recover from the attack. If subsequently the virus again seeks to 

 enter his body it will be met by the antibodies already formed, ready to 

 neutralize or otherwise destroy it. We say that such a person has become 

 immune to smallpox. 



As everyone knows, it is not necessary actually to contract smallpox to 

 become immune to it. By the process of vaccination a little of the virus, 

 so treated that it has lost its virulence and cannot cause the disease, is 

 introduced into a person's blood stream. The body reacts against the virus, 

 forming antibodies which will later prove of protective value if an active 

 virus is encountered. 



The protective mechanism just mentioned is called into action not only 

 by harmful organisms but also by any foreign substance of a protein na- 

 ture. A substance which will induce the formation of antibodies is called 

 an antigen. If, for example, serum (the fluid portion) of the blood of a 

 horse is inoculated into a rabbit, the latter's defense mechanism will be 

 called into play, resuUing in the formation of antibodies against horse 

 serum. The antibodies are themselves protein substances, and many of 



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