Sensitized Serum 113 



to earlier experiments from which it was found that the 

 invasion of the body by foreign substances sometimes pro- 

 duces profound modifications in the blood. 



Sensitized Serum 



If some white of an e^^ is injected mto the blood of a 

 rabbit, or another mammal, no immediate reaction follows. 

 If, however, a week or ten days later a second injection of the 

 albumen is made, very violent symptoms appear. We say 

 that the rabbit has been sensitized to the given albumen. If 

 another protein is injected this second time, there is no re- 

 action. The animal has been sensitized to a very specific 

 substance. The change in the blood is apparently the sort 

 of thing that makes possible resistence to disease infection. 

 It is the production by the body of something that counter- 

 acts the foreign protein, an anti-body, perhaps in some cases 

 an antitoxin (see page 61). On the other hand, this same 

 process of life reacting to attack gives us the distressing and 

 sometimes serious conditions known as hay fever and bron- 

 chial asthma. In this case, the body becomes sensitized to 

 a particular kind of protein (for example, the pollen of 

 golden rod, the fur of the cat, and so on) , and is subse- 

 quently irritated or upset when the same kind of protein 

 is again introduced. 



It may be difficult to see how a given process, which is 

 properly described as an adaptive response to a chemical at- 

 tack, can save life in one case (as in diphtheria or typhoid 

 fever) and bring about distress in another (as in asthma or 

 strawberry rash) . If some blood of the sensitized animal is 

 allowed to stand until a clot is formed, there is left a clear 

 liquid called the serum. It is in this serum that we find the 

 changed condition which is important in our investigation. 

 The serum of the sensitized animal cannot be distinguished 

 from the serum of the normal animal by its appearance or 

 by any known chemical test. Suppose, however, that we 

 add to each sample a drop of fluid containing the specific 



