328 DISEASES OF SWINE 



toxophore are cast off and sent out to meet the oncoming horde of 

 germ toxins. Other cells that have not yet been attacked are also 

 enlisted to aid in the battle, and soon the blood-stream is swarming 

 with these newly formed receptors, each looking for a toxin to 

 attack and disable. These receptors in the blood-stream are 

 decided workers against the poisonous bodies of the germ invaders, 

 and they are, accordingly, classed as antibodies. It is due to the 

 presence of these antibodies in large numbers that the attack of 

 the disease germs is finally checked and recovery takes place. 

 In some cases the germs are too quick for the body cells, and flood 

 the blood-stream with toxins so rapidly that all the cells are de- 

 stroyed before they have had a chance to form antibodies in suffi- 

 cient number to repel the invasion. In such cases the outcome is 

 fatal and death takes place. 



In those cases that do recover the cells of the body have been 

 taught a valuable lesson. They have learned that they need pro- 

 tection against these particular germs, and they also find that they 

 have acquired the power of manufacturing a defensive body that is 

 capable of affording this protection. The cell, therefore, concludes 

 that it is the part of wisdom to see that the blood-stream is at all 

 times kept supplied with a plentiful number of antibodies. 



It is due to the continuous presence of these antibodies, or 

 germ fighters, in the blood-stream that the animal maintains a de- 

 fensive power against this particular disease. This defensive 

 power is what is known as immunity. Immunity, then, is the 

 result of the presence in the blood-stream of a large number of 

 defensive agents known as antibodies. The permanent character 

 of this form of immunity is explained by the fact that the cells, 

 which have learned their lesson in the school of actual experience 

 in germ warfare, never allow themselves to again become open to 

 attack by these same germs. Just as the nation which is caught 

 unprepared for war learns its bitter lesson, and forever after keeps 

 up a strong army and navy for defense against an invader, so the 

 body learns its lesson, and maintains a strong army of defense 

 against subsequent attack. 



An immunity acquired in this manner is spoken of as an active 

 immunity, due to the fact that the cells of the body took an active 

 part in its production, and continue to actively take part in main- 



