552 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



to exclude normal agglutinins when testing the serum of the infected 

 case for the purpose of diagnosis. 



Normal Precipitins. No normal precipitins for bacteria occur in the 

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

 For example, human serum will precipitate the monkey serum. These 

 substances will be discussed in detail under acquired immunity. 



ACQUIRED IMMUNITY. Acquired immunity is that resistance which is 

 acquired after having an infection, being artificially inoculated with the 

 etiological microorganism of the infection (vaccination), or by being 

 inoculated with the products remaining in the body after an infection, 

 either natural or artificial, has taken place. 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 produced in active immunity are inoculated into other animals 

 the immunity conferred is called passive immunity. 



Active Immunity. Active immunity may be produced artificially in 

 the following ways: by the injection of living bacteria, by the injection 

 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), by the injection of bacteria or 

 bacterial products which in no way are related to the bacterium against 

 which immunity is conferred (p. 469) . 



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

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

 dal substances (lysins). As a result of the injection of dead bacteria, 

 the opsonins are increased in the blood. As a result of the injection of 

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

 antitoxins are produced. As a result of the injection of the disintegra- 

 tion products of bacteria, namely, endotoxins, bactericidal substances 

 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 in- 

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

 this type of artificially produced acquired immunity the body cells do 



