INFECTION AND IMMUNITY 343 



It is a mistake to think of immunity with reference 

 to physical similarity or dissimilarity. The glanders 

 bacillus finds the white mouse immune, but the field 

 mouse the most susceptible of all animals. The differ- 

 ences between these two species of mice do not appear 

 great. It is also impossible to connect the immunity of 

 the white mouse with any habituation it or its ancestors 

 may have enjoyed. 



It is difficult to make generalizations concerning 

 natural immunity because of its relative character. 

 What do we mean when we say that an animal is immune? 

 The rat has been declared immune against diphtheria; 

 it resists infection, is not injured by several thousand 

 times as much filtered culture (toxin) as will kill a guinea- 

 pig, but may be killed if the quantity of toxin injected 

 into it be out of all proportion. The case of the hen is 

 similar; it resists infection with the tetanus bacillus, 

 resists the injurious effects of reasonable amounts of its 

 toxin, but may be killed if the quantity of toxin injected 

 into it be extreme. The mongoose suffers but little 

 from a snake bite such as would destroy a rabbit and 

 is therefore said to be immune against venom but it 

 may be killed if overwhelmed by the poison. Thus we 

 see that the tolerance is in many cases relative and not 

 absolute. 



In acquired immunity we have to do with an ability to 

 endure intoxication or to resist infection that has been 

 acquired by an individual of a naturally susceptible 

 kind. It is a peculiarity of the individual not shared 

 by his kind. It is acquired through circumstances arising 

 during his own lifetime, and is neither inherited from 

 his antecedents nor transmitted to his descendants. 



Numerous examples of acquired immunity occur in 

 the experience of most persons. In childhood most of 

 us pass through the throes of measles, chicken-pox, 

 whooping-cough, scarlatina and mumps, and these 

 diseases we do not expect to have again, because they 

 usually leave acquired immunity that persists through- 



