THEORIES OF IMMUNITY 25 



When a certain quantity of such a product is injected into 

 the blood of an animal it is found at the end of twenty-four 

 hours that one part has been able to penetrate into the tis- 

 sues and has been fixed in the cells, a second part has been 

 eliminated, while a third part is still found to circulate in 

 the blood. Thus in the case of disodoluargol (the disodium 

 salt of dioxydiaminoarsenobenzene-antimonious-silver-bro- 

 mide) ; if 10 eg. are injected into a rabbit's vein, after twenty- 

 four hours there will be found of it about a quarter in the 

 excretions and a quarter in the blood and we ought to con- 

 clude that the other two quarters have been absorbed and 

 fixed by the organs and tissues. Three-quarters of the 

 injected product have thus been able to traverse cellular 

 membranes within twenty-four hours while the final quarter 

 composed of larger granules has not been transformed into a 

 dialyzable product. 



For each colloid the proportions may be different but for 

 the colloids of biologic origin we are unable to evaluate them 

 accurately because we do not know the chemical constitution. 

 We cannot conceive an idea of their quantity by biologic 

 reactions which are always uncertain. But we do know with 

 certainty that, as in the case of the arsenobenzenes, albumins, 

 like the toxins injected into the blood, are partly fixed by the 

 tissues some hours or some days after the injection; because 

 we see these tissues react with characteristic symptoms and 

 we know that no part of these products circulates for a long 

 time in the blood without being modified. Toxins generally 

 remain longer in circulation than albumins (white of egg, 

 serum, etc.). Thus we may conclude that they are more 

 fluid and less colloidal, than these latter. 



We have seen above that all colloidal substances on which 

 experiments have been made up to the present time have, 

 from a biological point of view, another character in common: 

 injected into the blood or under the skin of an animal, they 

 cause, after a variable incubation period, the formation of 

 " antibodies in excess," that is to say, of products which 

 exert certain specific actions on themselves. 



For want of a better term all these substances have been 

 grouped as " antigens" precisely because they provoke the 



