IMMUNITY AND SUSCEPTIBILITY 697 



are present in the blood serum of some animals. It is necessary, of 

 course, to exclude normal agglutinins when testing the serum of the 

 infected case for the purposes of diagnosis as will be mentioned later. 



Normal Precipitins. — No normal precipitins for bacteria occur in the 

 serums of animals. Precipitins for various blood sera, however, do occur. 

 For example, human serum wiU precipitate the monkey serum, etc. 

 These substances will be discussed in detail under acquired immunity. 



Acquired IMMUNITY.^-Acquired immunity is that resistance which 

 is acquired after having an infection or from artificial inoculation 

 with the etiological microorganism of an infection or from inocula- 

 tion with the products remaining in the body after infection, whether 

 natural or artificial. Acquired immunity may be divided into two 

 classes, namely, active and passive. Active immunity is that immunity 

 resulting from an infection or vaccination. In it the body cells react 

 and give rise to the formation of antibodies. When antibodies pro- 

 duced in active immunity are inoculated into other animals the im- 

 munity conferred is referred to as passive immunity. 



Active Immunity. — ^Active immunity may be produced artificially 

 in the following ways: By the injection of living bacteria; by the injec- 

 tion of bacteria of reduced virulence; by the injection of dead bacteria; 

 by the injection of the secretory and excretory products of bacteria 

 (toxins, etc.); by the injection of the disintegration products of bacteria 

 liberated after the death of the cells (endotoxins) ; and by the injection 

 of bacteria or bacterial products which in no way are related to the bac- 

 terium against which immunity is conferred. 



As a result of the injection of living bacteria in small amounts or of 

 bacteria of reduced virulence the body cells react and produce bacteri- 

 cidal substances (lysins, etc.). As a result of the injection of dead bac- 

 teria, the opsonins are increased in the blood. As a result of the injec- 

 tion of the secretory and excretory products of the bacteria, namely, 

 toxins, antitoxins are produced. As a result of the injection of the dis- 

 integration products of bacteria, namely, endotoxins, bactericidal sub- 

 stances are produced. In cases where bacteria or bacterial products, 

 which are in no way related to the bacterium against which immunity 

 is conferred are injected, it is probable that bactericidal substances are 

 produced. This condition only occurs in rare instances. 



Passive Immunity. — Passive immunity may be conferred by the 

 injection of antitoxins, and by the injection of bactericidal substances. 



