CHAPTER IX. 

 THE DIFFERENT TYPES OF IMMUNITY. 



WE have seen in the foregoing chapter how satisfactorily Ehrlich's 

 theory accounts for the formation and specific action of the anti- 

 bodies, and thus for the origin and mode of action of some of the 

 most important defensive factors of the animal body. Upon this 

 basis we may now also take up for consideration some of the more 

 general aspects of the problem of immunity. 



When we speak of immunity in the biological sense, we under- 

 stand thereby the existence of a certain resistance toward dele- 

 terious influences. This may be directed against a large number 

 of factors, such as the action of various drugs and chemicals, the 

 harmful effect of atmospheric conditions, attack by other animals, 

 various degenerative influences arising from within the body, infec- 

 tions with vegetable or animal parasites and the absorption of their 

 products of metabolism or degeneration, etc. From a medical 

 standpoint, of course, these latter influences interest us particularly, 

 and in the following pages we shall devote our attention to the 

 subject of immunity from this standpoint more especially. 



Natural Immunity. The very fact that animal life is possible at 

 all, surrounded as we are by organisms which under certain con- 

 ditions can invade the body and cause its destruction, shows in 

 itself that every individual must possess a certain degree of natural 

 immunity. * Staphvlococci are thus found not only on the outer 

 surface, but even in the deeper layers of the skin without giving rise 

 to any disturbance J pa Jhpgenic pneumococci, streptococci, and even 

 diplitheria bacilli may be present in thTfauces without producing dis- 

 ease* the intestinal canal is inhabited by untold millions of bacteria, 

 some of them of pathogenic character, which apparently produce 

 no deleterious effects, and so on. Apparently the individual who 

 normally harbors all these various organisms is immune to the 

 corresponding infections. 



The picture, however, changes very materially, if in some manner 



