THEORIES OF IMMUNITY. 



occurring in the production of immunity are 

 closely related to those that go on under the 

 ordinary conditions of tissue metabolism, and the 

 only way in which this can be settled is by the 

 determination, in the first place, of what the nor- 

 mal functions of the cells are ; and in the 

 second, whether substances similar to the anti- 

 bodies exist under normal conditions in the body. 



Inquiry in the first direction has as yet left 

 much obscurity to be cleared away, and the exact 

 functions of the leucocytes, or of the cells from 

 which they are derived, are not clearly understood. 

 As to the second point, so many investigations 

 have seemed to show the facts that it can hardly 

 be doubted that substances closely allied to if not 

 identical with both complement and immune body 

 do exist in the sera of normal animals — it is, 

 however, disputed as to whether they exist free 

 in the normal blood stream. 



If they do so exist they must have some 

 normal function, for it is not to be supposed 

 that they exist for the sole purpose of taking 

 part in the specific reactions of immunity when 

 such necessity may arise. They must take some 

 part in normal tissue metabolism; and in attempt- 



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