THE MICROBES THAT MAKE US SICK 83 
called antibodies . The making of these anti¬ 
bodies is the reason why a person who has had 
measles or whooping cough, for instance, does 
not easily take the disease again. 
The discovery of antibodies has also led to 
the inventing of ways to protect a person from 
the attacks of microbes, and to help cure the 
disease when it has appeared. It has been 
found that the antibody substance formed in 
the blood of an animal when attacked by a 
certain microbe can be placed in the body of 
a human being and will there protect him 
against the same microbe. Or, if he is already 
ill with the disease, it will help to kill the 
microbes in his body and bring about his 
recovery. When this is done, the substance 
transferred from the animal to the human 
being is called an antitoxin. We have already 
mentioned some of these antitoxins. 
To obtain diphtheria antitoxin, the poison 
produced by the diphtheria germ is put into 
the blood of a horse. An antibody, or anti¬ 
toxin, then grows in the horse’s blood to fight 
What are 
antibodies? 
