20 THEORIES OF IMMUNITY 



better understand the true significance of facts in daily 

 experience and to better group these facts in order to con- 

 struct a general idea which in its turn will point the way 

 toward new researches. In the discussion which follows we 

 shall try to recall the merits of the two theories of Metchnikoff 

 and Ehrlich and of the experimental researches which these 

 have inspired. We shall try to interpret the results of these 

 researches in the light of new facts and to draw from the total 

 of these studies conclusions which are necessarily logical. 



Metchnikoff's theory of immunity attributed the defensive 

 mechanism of the organism against pathogenic germs exclu- 

 sively to the leukocytes. Leukocytes engulf the germs, 

 digest them and there results an overproduction of a specific 

 digestive ferment which renders this digestion more and more 

 easy and active. When all the microbes are thus destroyed, 

 there is cure and the organism remains immunized against 

 the same germ because the leukocytes which have learned 

 to digest these germs retain this new property for a certain 

 time and transmit it by heredity to their descendants so that 

 they also will be able to attack a small quantity of the same 

 parasites in case the organism again becomes spontaneously 

 infected. 



We may easily understand the destruction of the microbes 

 in the interior of the leukocytes. The role of these latter in 

 the defense of the organism is certainly very important but 

 we must recall that if the phagocytic theory contains one 

 part of the truth it cannot include the whole problem of 

 pathogenicity and immunity because the struggle between 

 microbes and organism does not take place exclusively in 

 the blood. If we must assume that it is the microbe which 

 is the origin of disease and that its destruction by the white 

 blood corpuscles can lead to cure, it is by no means less 

 certain that it is not only the microbe body but also and often 

 principally the soluble products or poisons which it secretes 

 during its life or which are allowed to diffuse after its death 

 that by acting not only on the leukocytes but also on a large 

 number of other cells, produce the lesions and symptoms 

 characteristic of each disease. 



Pathologic manifestations can be only the results of reac- 



