IMMUNITY AND ANAPHYLAXIS 197 



tion, each leucocyte probably having a definite quantity 

 within its substance. 



Aggressins. The great susceptibility of some species 

 of animals to infection by certain bacteria, while their 

 serum nevertheless possessed marked bactericidal power 

 against these bacteria, suggested to Bail the theory that 

 these bacteria secrete definite substances, which protect 

 them against phagocytosis. Such substances he called 

 " aggressins," and they are therefore antagonistic to the 

 opsonins. They are probably not produced in test tube, 

 or only to a slight degree. He based this theory on two 

 observations, namely, that sub-lethal doses of bacteria, 

 injected along with a small quantity of " aggressins" were 

 rapidly fatal ; and that animals could be immunized against 

 the corresponding bacteria by the injection of the aggressins. 

 The aggressins were got, as detailed on page 177, in the 

 exudate into a serous cavity of an animal killed by the 

 injection of a dose of a particular bacterium into the serous 

 cavity, and from which exudate the bacteria are carefully 

 removed. This theory has been attacked on the ground 

 that the aggressins are merely the bacterial toxins, probably 

 endotoxins, liberated in the living body. The fact that 

 such exudates are usually cellular, along with what has 

 been said above of the action of leucocyte extract in some 

 infections, tends to confirm this criticism. In fact, it may 

 be put this way : When there is a high natural resistance 

 to a bacterium, the alexines and opsonins are able to 

 overpower it in all average infections and prepare it for 

 phagocytosis and subsequent destruction. On the other 

 hand, where the natural resistance is low, these agents do 

 not succeed in preventing the growth of the bacterium, 

 which in its growth elaborates various substances, some 

 of which reduce the resistance still further, and so progres- 

 sive infection results. If this infection is not too severe, 

 the immunizing apparatus throughout the body, stimulated 

 by the diluted toxins (extra- or infra-cellular) present in 

 the blood stream, produces an excess of antibodies and 

 anticells (phagocytes), and in this way may attain the 

 objective of active immunization. If the infection is too 

 acute, paralysis of the immunizing apparatus is the result. 



