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 ioo 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 
occasion, 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 
Phago¬ 
cytosis 
