112 THE ACUTE SELF-LIMITED INFECTIONS 



germs on broth, made in a manner found most suitable 

 for its development. The broth is freed of bacterial 

 bodies and injected into horses. This animal is chosen 

 for its size and freedom from disease affecting humans, 

 and because large quantities of material may be 

 injected and much blood withdrawn without harm- 

 ing the beast. The horses receive under the skin 

 gradually increasing amounts of this toxic broth until 

 they are able to withstand huge quantities, many 

 times the dose necessary to kill them if given at first. 

 They are then considered to have some neutralizing 

 substances for this toxin. This neutralizing property 

 is known to be in the blood serum. The horse is then 

 bled, and the serum separated from the red blood 

 cells. It is tested against the original toxin used for 

 making the injections. This is done by mixing the 

 two in definite parts, allowing the mixture to stand 

 a few minutes, and injecting it into guinea-pigs. By 

 appropriate technic the number of " units" is deter- 

 mined. A "unit" is that quantity of horse serum, or 

 antitoxin, which will neutralize 100 times the smallest 

 quantity of toxin necessary to kill a guinea-pig weigh- 

 ing 250 grams (8 ounces). 



The horse-serum antitoxin has now a value for clinical 

 purposes, as the quantity to be given can be controlled. 

 Newer methods have permitted the refinement and 

 concentration of this antitoxin, so that there is now 

 less inconvenience in giving it. The dose for treatment 

 varies from 1500 to 5000 units by injection under the 

 skin. In bulk this may be less than a teaspoonful. 

 For immunizing purposes, that is, to protect persons 

 exposed but not yet suffering from the disease, from 



