312 Microbes and You 



The Humoral Theory of Immunity 



Toward the end of the nineteenth century Buchner, von 

 Behring, and others reported that the blood serum of persons who 

 had been vaccinated, or who had recently recovered from certain 

 diseases, contained protein fractions called antibodies. Such 

 humoral (fluid) immunity appeared to be the clue for which re- 

 search workers had been looking. Our present beliefs with respect 

 to immunity are largely dependent upon these antibodies as our 

 explanation of degrees of resistance to disease. The types of these 

 antibodies will receive more attention in later discussions in this 

 chapter. ^ -^ 



TYPES OF IMMUNITY 



Whenever a person or a lower animal plays an active part in 

 building up his own resistance to microbial diseases, it is termed 

 ACTIVE IMMUNITY. But if antibodies produced in other persons or 

 in lower animals are injected into an individual who has played no 

 active role in the manufacture of these protective substances, this 

 is PASSIVE IMMUNITY. Activc immunity is longer lasting than 

 passive immunity. 



Having been bom the particular species of animal that we are 

 endows us with a natural resistance to certain diseases and natural 

 susceptibility to other diseases. Man is apparently the only animal 

 susceptible to typhoid fever. Dogs can contract distemper, but 

 man is refractory. While immunity is a relative thing, natural 

 resistance to certain microbes because we are human beings is a 

 complete immunity. 



Acquired immunity can be induced either bv recovering from a 

 disease, or by injecting the microbial agent or its products. There- 

 fore there is both a natural acquired immunity and an artificial 



ACQUIRED IMMUNITY. 



When we are born we acquire for relatively short periods of 

 time an immunity to at least some of the diseases for which our 

 mothers have antibodies. Even though the cellular constituents 

 of the mother's blood are not common to the circulation of the 



