ACQUIRED SUSCEPTIBILITY. 131 



ity disappears. That of the parent, however, per- 

 sists for a much longer period; it is "active" in 

 character, as explained later. Even in a given 

 family, however, there are often marked variations 

 in susceptibility and resistance. One child in a 

 family may contract scarlet fever, while another, 

 living under exactly the same conditions, may 

 escape it. 



Susceptibility also is often acquired in a more 

 or less evanescent way. The resistance of *"' fa jj- 

 vidual may vary greatly at different times and 

 under different conditions. These are accidental, 

 acquired states such as may be occasioned by 

 exhaustion, hunger, exposure to cold and other 

 unhygienic conditions. 



Recently, the existence of a specific acquired Acquired s 

 susceptibility to various proteid substances and 

 bacterial products has been determined by experi- 

 mentation. An injection of serum or various pro- 

 teids into the proper animal renders it extremely 

 susceptible to a second injection; and a person 

 who is suffering from a particular infection shows 

 unusual sensitiveness to the products of the organ- 

 ism causing the infection, as shown by the tuber- 

 culin reaction of Koch. Such animals and indi- 

 viduals are said to have been sensitized. This sub- 

 ject will be discussed later under "Anaphylaxis." 



Many of these facts were familiar long before 

 anything was known regarding the principles on 

 which they depend. Subsequent to the discovery 

 of some of these principles (to be considered 

 later), it became convenient and necessary to rec- 

 ognize other special types of immunity, although 

 any type which can be conceived must still find a 

 place under either natural or acquired immunity. 



