ANTI-BODIES AND IMMUNITY 197 



ditions of disease. The invading bacteria, in some cases, pro- 

 duce poisons which destroy cells of the body and phagocytes; 

 with their destruction the contained digestive fluids or chemicals 

 are liberated; these in turn react on the bacteria and kill them. 

 Thus, if rabbits are inoculated with the bacilli of anthrax, the 

 parasites multiply in the blood, over-run every organ of the 

 body and ultimately kill the rabbit. If, however, the bacilli 

 are placed in rabbits' blood that has been drawn out into a test- 

 tube, the bacteria are killed (Nuttall and Buchner). This 

 result, as it is usually interpreted, is due to the death and disin- 

 tegration of phagocytes, whereby some chemical substance 

 (called alexine by Buchner), which is fatal to the bacteria, is 

 liberated from the disintegrating protoplasm. Alexine is 

 supposed to be the same substance which brings about digestion 

 of bacteria ingested by living phagocytes. 



Some types of bacteria in the blood are attacked and killed 

 by the phagocytes. This was demonstrated by Metschnikoff 

 who, using a strain of bacteria which are thus killed in the blood, 

 enclosed some in collodion sacs which he placed in the body cavi- 

 ties of different animals. These sacs allowed the free inter- 

 change of fluids, including bacterial products and various sub- 

 stances contained in the blood, but the bacteria themselves 

 could not get through, nor could the phagocytes reach the 

 bacteria which lived and thrived in the collodion sacs. 



Again, some types of bacteria in the blood are not touched 

 by phagocytes under ordinary conditions, but if animals con- 

 taining such bacteria are inoculated with phagocyte-free blood, 

 which is immune to these bacteria, the phagocytes immediately 

 devour them. Something in the serum has produced a change 

 in the bacteria which, while it leaves the bacteria uninjured so 

 far as their vital processes are concerned, renders them suscep- 

 tible to attack by phagocytes. Wright, who discovered this 

 phenomenon, gives the name opsonin to the substance which 

 makes bacteria susceptible to phagocytes (opsono I prepare 

 the food). 



(2) Anti-bodies and Immunity. While phagocytes are thus a 

 potent physiological adaptation for protecting the organism 



