INFECTION, SUSCEPTIBILITY, AND IMMUNITY 27 



phagocytosis depended upon the presence in the blood 

 of substances which they called opsonins, which so act 

 upon live bacteria as to make them fit food for the phago- 

 cytes, thus producing just the opposite effect from that 

 of the aggressins. They found that these opsonins differ 

 for the different varieties of bacteria, that an opsonin 

 for one variety of germ had no effect upon germs of 

 another variety. 



Bacterial Vaccines. They further found that by 

 injecting dead cultures of a germ the opsonins for that 

 germ were increased, and phagocytosis stimulated ac- 

 cordingly. These dead cultures prepared for such use 

 were called bacterial vaccines. 



Antitoxin. When any plant organism grows on a [ 

 certain soil for any length of time it produces substances 

 detrimental to its own life and growth. The farmer 

 recognizes this in his rotation of crops. So bacteria 

 grown too long on the same artificial media cease to 

 flourish and even die. In the animal body this doubtless 

 occurs, but of more importance is the fact that there 

 are produced by the body cells antibodies or substances 

 antagonistic to bacterial growth and antidotal to their 

 poisons. Whether produced by the phagocytes or by 

 the fixed tissue cells, or by both, is not definitely known, 

 nor is it important to us. The fact remains that there 

 is produced in the course of certain diseases certain sub- 

 stances called antitoxins, which may be recovered from 

 the blood of the animal which has been the subject of the 



