BLOOD AND H^MOPOIETIC ORGANS 89 



mechanism and meaning of this physiological leuco- 

 cytosis is obscure, and it does not seem to be of much 

 importance. Its occurrence must be borne in mind, 

 however, when one is estimating the leucocytes in 

 disease, and the time of the blood examination chosen 

 so as to avoid it. 



The average duration of life of a leucocyte is quite 

 unknown, but it is almost certainly much less than that 

 of a red cell, and may not amount to more than a few 



FIG. 2. DIFFERENTIAL PERCENTAGE COUNTS THROUGHOUT LIFE. 

 (AFTER CARSTANJEN.) 



days. Hence any theory of immunity which is based 

 upon the * education ' of individual leucocytes in dealing 

 with bacteria must be regarded as very unlikely to be 

 correct. 



Leucocytes probably perish everywhere. We know, 

 indeed, that many of them are destroyed under patho- 

 logical conditions at the seat of infections, and subse- 

 quently are themselves * englobed ' by mononucleated 

 phagocytes, thus falling victims to the same fate which 



