ANTI-BODIES AND IMMUNITY 193 



disease. The invading bacteria in some cases produce poisons 

 which destroy cells of the body and phagocytes; with their de- 

 struction the contained digestive fluids or chemicals are liber- 

 ated; 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 ulti- 

 mately kill the rabbit. If, however, the bacilli are placed in rab- 

 bits' blood that had been drawn out into a test-tube the bacteria 

 are killed (Nuttall and B uchner) . This anomaly is explained by 

 the liberation into the blood plasm of chemical compounds 

 (alexine of Buchano) derived from the broken-down protoplasm 

 of phagocytes, which is fatal to the bacteria, just as digestive 

 fluids of the living phagocytes are fatal to engulfed bacteria. 



Ordinarily bacteria in the blood are attacked and killed by the 

 phagocytes. This was demonstrated by Metschnikoff who, 

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

 them in collodion sacs which he placed in the body cavities of 

 different animals. These sacs allowed the free interchange 

 of fluids, including bacterial products and various substances 

 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 treated with immune serum 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 susceptible 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 

 against disease, they do not form the sole means of protection. 

 The phenomena of immunity furnish another and more subtle 

 physiological adaptation. 



