CHAPTER XX. 



VACCINATION AND IMMUNITY. 



WE have in the foregoing chapter tried to show that owing 

 to the presence in the normal blood and tissues of a living 

 animal of some chemical substances varying in the different 

 species and inimical to particular pathogenic organisms, the 

 latter when introduced into the tissues of the particular 

 species, cannot thrive, and that it is for this reason that the 

 animal is not susceptible to the corresponding disease. Now, 

 how do we explain the fact that a human being or an animal 

 having been once the subject of a particular infectious disease, 

 becomes thereby in some cases unsusceptible to a second 

 attack ? The oldest and perhaps the most favoured theory to 

 explain this immunity is that which assumes that during the 

 first attack the organisms growing in the body consume, or 

 are instrumental in eliminating or destroying, some chemical 

 compound necessary for the existence and multiplication of 

 the organism. As soon as this substance has become con- 

 sumed or destroyed the organisms cannot further multiply, 

 and therefore the disease ceases ; and further, that owing to 

 the subsequent absence of this same chemical compound, a 

 new infection by the same organisms is not possible, i.e. the 

 individual is protected. Thus this theory puts the case on a 



