United States Bureau of Plant Industry 



NATURAL IMMUNITY IN PLANTS 



One variety of tobacco was grown between rows of other varieties. All the plants 

 were sprayed with fluid containing spores of black shank, a fungus disease of to- 

 bacco. Among plants, as among animals, individuals and strains of individuals differ 

 from others in the degree to which they are susceptible to particular parasites or 

 diseases 



Immunity and Susceptibility Individual variation includes great dif- 

 ferences in sensitivity to particular substances. Some people catch colds 

 more easily than others. Some more frequently have boils or pimples. 

 There are also racial differences. Thus dark-skinned races are less suscep- 

 tible to malaria and to hookworm than white races. On the other hand, 

 white races are less susceptible to tuberculosis and measles than dark races. 

 Again, human beings are quite immune to diseases that are serious or even 

 fatal to birds or cattle (see illustration above). 



Such immunity is called natural immunity, and is inherited. In many 

 cases it probably depends upon the chemical peculiarity of the blood. In 

 others it depends upon the quick response of the living cells to poisons or 

 to other products of bacteria. But such natural immunity is not absolute; 

 that is to say, it may be weakened or destroyed by various conditions. The 

 quantities of certain antibodies in human blood can be tremendously in- 

 creased by the inoculation of suitable foreign substances. This is the basis 

 for the various kinds of artificial immunization, which are popularly called 

 "vaccination". 



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