CHAPTER XXII. 



IMMUNITY. 



Introductory. — By immunity is meant non-susceptibility to a 

 given disease or to a given organism, either under natural 

 conditions or under conditions experimentally produced. The 

 term is also used in relation to the toxins of an organism. 

 Immunity may be possessed by an animal naturally, and is then 

 usually called natural immunity, or it may be acquired by an 

 animal, either by its passing through an attack of the disease, or 

 by means of artificial inoculation. It is to be noted that man and 

 the lower animals may be exempt from certain diseases under 

 natural conditions, and yet the causal organisms of these diseases 

 may produce pathogenic effects when injected in sufficient 

 quantity. Immunity is, in fact, of very varying degrees, and 

 accordingly the use of the term has a relative significance. 

 This is not only true of infection by bacteria, but of toxins also 

 — when the resistance of an animal to these is of high degree, 

 the resistance may in certain cases be overcome by a very large 

 dose of the toxic agent. On the other hand, even in cases where 

 the natural powers of resistance are high, these can be still 

 further exalted by artificial means — that is, the natural immunity 

 may be artificially intensified. 



Acquired Immunity in the Human Subject. — The following 

 facts are supplied by a study of the natural diseases which affect 

 the human subject. First, in the case of certain diseases, one 

 attack protects against another for many years, sometimes 

 practically for a lifetime, e.g., smallpox, typhoid, scarlet fever, 

 etc. Secondly, in the case of other diseases, e.g., erysipelas, 

 diphtheria, influenza, and pneumonia, a patient may suffer from 

 several attacks. In the case of the diseases of the second group, 

 however, experimental research has shown that in many of 

 them a certain degree of immunity does follow ; and, though 

 we cannot definitely state it as a universal law, it must be 

 considered highly probable that the passing through an attack 



