FUNCTIONS OF THE BLOOD 277 



bacterial stimulation produce what is thought to be a third sub- 

 stance, agglutinin, which causes the bacteria to clump together, 

 becoming thus more subject to the action of the phagocytes or 

 complements. The development of these various bodies is the 

 process of immunization. 



Specific Nature of Opsonins and Immune Bodies. A very 

 interesting fact about the bodies which the living cells develop 

 under bacterial stimulation is that their action is very specific. 

 The opsonins which are developed in an individual under the 

 stimulus of invading typhoid bacilli increase the tendency of the 

 phagocytes to ingest those particular organisms and no others. 

 Similarly immune bodies developed by the pneumococcus enable 

 the complements of the blood to attack those bacteria and no 

 others. For this reason immunity against one kind of infection 

 does not protect the Body against other kinds. 



Antitoxin. Beside the destruction of the invading bacteria 

 it is necessary, before the Body is cured of an infection, that 

 the toxins produced by the rapidly multiplying organisms be neu- 

 tralized. This neutralization of poison is a simpler process than 

 the destruction of formed elements as carried on by the phago- 

 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 cooperation 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 fifteen years. 



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 sur- 

 face where they develop and whence they discharge their toxin 



