INFECTION AND IMMUNITY. 39 



In other words, active immunity has been established, 

 in the horse. At the end of three or four months the 

 animal is bled to the amount of five or six quarts, and 

 the blood is set aside to clot. In the serum that sep- 

 arates from the clot are the same substances that pro- 

 tected the horse from the diphtheria poison. This is 

 the diphtheria antitoxin. It is standardized by de- 

 termining the smallest amount of antitoxin that will 

 neutralize 100 times the fatal dose of toxin for a 

 guinea-pig weighing 250 grams. This amount is 

 called the antitoxin unit, and enables us to measure 

 the dose of antitoxin. 



What the nature of these substances is that en- 

 ables us to resist infection is not known, and the way 

 in which they act is built up on theory that is com- 

 plicated and difficult to understand. It is sufficient for 

 us to know that soon after infection occurs the body 

 tissues and fluids begin to protect themselves against 

 the invading bacteria and their poisons. The first 

 defense is made by the white blood-corpuscles, or 

 leucocytes, the scavenger cells of the blood. They 

 are attracted in great number to> the point of infection 

 and destroy the invading bacteria by taking them into 

 their cell bodies and digesting them. The fate of 

 infections depends many times on the defense of the 

 phagocytes; if they are sufficient for the needs of the Phago- 

 occasioii, the infection is checked and localized ; if they 

 are not, the infection extends and may become general. 



The body, however, does not rely entirely on the 

 phagocytes for protection. Infection stimulates the 



