THE IMMUNITY REACTION 231 



destroyer of life at the time of the Spanish War, there 

 was only a handful of well-authenticated cases in the 

 American army during the Great War, is a practical 

 application of this reaction by which the power of the 

 phagocytes to engulf and destroy typhoid organisms is 

 greatly increased. 



The Immunity Reaction. — Many diseases, among 

 them smallpox, pneumonia, and in fact the majority of 

 the more prevalent infections, stimulate the body, in 

 which the disease has gained a foothold, to produce sub- 

 stances known as anti-bodies, which are capable of (a) 

 destroying foreign organisms or (6) of neutralizing the 

 poisons which they produce. This disease-resisting prop- 

 erty of the anti-bodies is merely a special and very 

 useful application of a general power that living pro- 

 toplasm has of getting rid of foreign proteins. It will 

 be recalled that foreign proteins, as such, rarely come 

 into contact with the living cells of the higher animals, 

 because all proteins that are taken in with the food are 

 digested into their constituent amino-acids before being 

 absorbed into the body fluids. It is possible, however, to 

 inject proteins directly into a vein, or they may obtain 

 access through infection, the bodies of disease-producing 

 organisms and the poisons they produce both being 

 largely protein in character. 



It is characteristic of the cells of the highly organized 

 animals that when foreign protein comes into contact 

 with them they react by producing anti-bodies. The 

 effect of the anti-bodies is to precipitate the protein or 

 dispose of it in some other way. An important feature 

 of the anti-bodies is that there is a particular sort for 

 each kind of protein, and when the cells of the body 

 react to the presence of a foreign protein, the anti-body 

 they produce is the particular one by which that protein 

 is disposed of. 



Diphtheria. — There are a few kinds of disease-pro- 

 ducing organisms, of which the diphtheria bacillus (Fig. 



