SCIENTIFIC BASIS OF SERUM THEORY 329 



taining it by constantly keeping up the supply of germ-fighting 

 bodies. 



It is due to the continued maintenance of the large supply of 

 these germ-fighting antibodies in the blood-stream that the serum 

 from this animal can be taken and used as a protective agent in the 

 body of another animal. This is the most important fact in con- 

 nection with immunity. It is on account of this power of protect- 

 ing a healthy animal by injection with antibodies derived from an 

 animal that has passed through an attack of the disease that the so- 

 called serum treatment of disease has been developed. Hog-chol- 

 era is only one of the many diseases in which this has been found 

 to be true. In connection with hogs, however, the immunizing 

 power of blood of an animal that has been through an attack of 

 cholera is of most importance, and it is with respect to the immuniz- 

 ing power of blood from a hog that has been through an attack of 

 cholera that we are most concerned in this book. 



In studying immunity and the manner of its production, a most 

 important discovery was made many years ago, in that it was found 

 that it was not necessary for an animal to have a severe attack of a 

 disease to develop immunity to future attacks by the same germ. 

 It was found that if a weakened culture of the same germ was in- 

 jected it produced a reaction which left the same degree of pro- 

 tective power as resulted from a severe attack by the disease itself. 

 This fact has been taken advantage of in human practice in vacci- 

 nation against small-pox, and is being used in a modified form in 

 the treatment of hog-cholera by the double or simultaneous 

 method. 



Hyperimmunization. — Investigators in the field of immunity 

 have found that an animal which possesses an immunity against 

 disease can stand an attack by the germs of that particular disease 

 in almost any number. Not only is this animal proof against the 

 usual fatal dose of these germs, but this same animal is able to 

 withstand the attack of these germs in enormous doses — even up 

 to one thousand times the usual number. 



The introduction into the body artificially of these large doses 

 of germs in an immune animal was found to have no bad effects, in 

 so far as producing sickness or disease is concerned. It was foun.d, 

 however, on testing the blood of an animal that had been given 



