310 THE HUMAN BODY 



cytes and complements. It is brought about in the Body, how- 

 ever, in much the same way. The foreign toxin stimulates the 

 cells of the Body to produce and pour into the blood an antitoxin 

 which neutralizes the toxin. Antitoxins, like opsonins and im- 

 mune bodies, are specific for the toxin which stimulated their 

 development. 



Immunity. An individual who has gone through an infection, 

 and by the co-operation of the forces described above has over- 

 come and destroyed the invaders with their harmful toxins, re- 

 tains for a long time afterward in his blood the special opsonins, 

 immune bodies, and antitoxins which were developed therein 

 during the course of the infection. He is, during this time, im- 

 mune toward that particular disease. The existence of this im- 

 munity has been known for centuries; its explanation is the result 

 of the work of the last twenty years. 



The duration of immunity varies greatly in different infections. 

 There is every degree from the extremely brief immunity toward 

 common colds, an immunity that apparently terminates with the 

 period of convalescence; to an immunity that is life long as in the 

 case of yellow fever. 



Carriers. A fact of interest, as well as of great moment in the 

 problem of caring for the public health, is that an occasional in- 

 fected individual, instead of destroying the invading organisms, 

 becomes so adapted to them that he continues in perfect health 

 with his Body swarming with pathogenic organisms. Such a per- 

 son is known as a carrier. He is a constant source of danger, since 

 the organisms he carries, and by which he is unaffected, are liable 

 to be transferred to some susceptible individual and cause severe 

 illness or even a widespread epidemic. 



The Use of Antitoxin in Disease. In some diseases, of which 

 diphtheria is the best known example, the bacteria do not spread 

 through the Body but take up their abode on a convenient surface 

 where they develop and whence they discharge their toxin into 

 the blood. Successful combating of such diseases requires only 

 that the toxin be neutralized. In course of time the bacteria will 

 reach the end of their development and die. 



The antitoxin for any particular kind of toxin will neutralize 

 it whether produced in the body which is infected or in some 

 other body from which it is transferred to the infected one. This 



