CHAPTER XXXIII 



ARTIFICIAL IMMUNITY IN THE CONTROL OF DISEASE 



Questions. 1. What happens when parasites invade the body ? 2. If 

 microbes are present nearly everywhere, how can infection be entirely 

 prevented ? 3. Why can one have certain diseases only once ? 4. How 

 can juices taken from a sick animal help one get well ? 5. Isn't vaccina- 

 tion dangerous ? 6. How can we tell that vaccination helps ? 7. How 

 can we tell that antitoxin helps ? 8. How does getting chilled make one 

 sick ? 9. Are diseases inherited ? 



255. Immunity and susceptibility. Individuals differ so greatly 

 that some are much more sensitive or much less sensitive to a 

 given substance or a given stimulus than are others. It is a 

 matter of common observation that some people catch cold more 

 easily than others ; some more frequently have boils or pimples ; 

 some are very susceptible to the effects of alcohol or some other 

 chemical ; some are more susceptible to typhoid or to diphthe- 

 ria. There are also racial differences. Thus, the dark-skinned 

 races are less susceptible to malaria and to hookworm than are 

 the white races ; on the other hand, the white races are less sus- 

 ceptible to tuberculosis and measles than are the dark races. 

 Moreover, there are important specific differences (that is, dif- 

 ferences connected with species). Hens are quite indifferent to 

 the action of morphin. Rabbits are insensitive to atropin. In 

 the same way human beings are quite insensitive to diseases that 

 are serious or even fatal to birds or cattle. Such immunity is 

 called natural immunity and is inherited. In many cases it 

 probably depends upon the chemical peculiarities of the blood 

 or other body juices. In other cases it probably depends upon 

 the quick reaction of the living cells to the poisons and other 

 products of the parasites. 



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