DISEASE-RESISTING FUNCTIONS OF THE BLOOD 311 



fact has made possible the development of the well-known anti- 

 toxin treatment. Animals, usually horses, receive doses of toxin 

 obtained by growing the bacteria on culture media in proper 

 vessels. These doses are small at first, but are gradually increased 

 as the animal acquires immunity. In course of time the blood of 

 an animal so treated contains large quantities of antitoxin. Con- 

 siderable amounts of blood can be withdrawn from animals the 

 size of horses without their suffering the slightest inconvenience. 

 It is thus possible to obtain abundant supplies of antitoxin. 



The methods of purifying antitoxin-containing solutions are 

 so perfect at the present time that no one should feel the least 

 hesitation at the prospect of its use. The percentage of deaths 

 from diphtheria has fallen from more than fifty to about two 

 since its introduction. 



Protective Inoculation. It has been found practicable in some 

 diseases, notably smallpox, to develop immunity by infecting the 

 Body with an organism which is not virulent enough to endanger 

 life but which produces immune substances that protect the Body 

 against the more virulent infection. On account of the specific 

 character of immunity this method can only be used where vir- 

 tually the same organism occurs in virulent and non-virulent 

 forms. 



The most hopeful path of progress at present toward the mas- 

 tery of disease is along the lines here indicated. We may look 

 forward confidently to a time when most if not all the acute in- 

 fections will be brought under medical control through applica- 

 tion of the principles of immunity. 



Anaphylaxis. Although in our discussion of immunity thus far 

 emphasis has been laid on it as a means of destroying disease germs 

 and their toxins, the fact is that the immunity reaction, considered 

 as a reaction, may manifest itself toward foreign protein substances 

 in general, whether they have any relation to disease or not. 

 Thus it is possible by injection of egg-white into the blood to cause 

 the Body to develop immunity toward that substance. 



In the development of immunity toward toxins of disease the 

 Body is under the influence of the toxin more or less continuously 

 for a time, and this continuous influence seems essential to the 

 normal progress of the immunity reaction. If, instead of such 

 continuous influence the Body receives a single dose of foreign 



