168 PRINCIPLES OF VETERINARY SURGERY 



side by side without fixing each othei^. On the other hand, 

 when the toxin molecule unites with the cell, it not only 

 diverts the receptors from their normal functions of nutri- 

 tion, but it actually poisons the cell so that the more toxin 

 introduced, the more receptors abound, and the greater 

 the injury to the cell. This injury the cell makes up by a 

 process of regeneration resulting in the manufacture of new 

 receptors which are reproduced in excess of the number 

 destroyed by the toxin : This excess may be so great that 

 the cell is over-filled with them — so over-filled that many 

 are cast off into the circulation and exist independently of 

 the cell. These cast-ofif receptors can still unite with toxin, 

 and thus become a means of protection to the cells. Let 

 us suppose that one thousand molecules of toxin get into 

 the body and one thousand free receptors unite with the 

 toxin. Then no toxin molecules reach the cells and no 

 cells are poisoned. This is immunity. 



These free receptors constitute our antitoxin and thus 

 become a means of protection to the cells. If we stimulate 

 a horse to produce free receptors by injecting him with 

 several non-fatal doses of tetanus toxin, we produce a state 

 of active immunity in his body. If we then bleed him and 

 inject his serum containing free receptors of antitoxin into 

 a horse stricken with tetanus, we find that the molecules 

 of tetanus toxin in the stricken horse are pulled away from 

 his tissue cells and become fixed to the free receptors of the 

 injected serum. We thus produce a passive immunity. 



As regards the structure of the antitoxin or cast-off 

 receptor, it remains the same as when attached to the cell, 

 i.e., it consists of one combining group only, the hapto- 

 phore. Ehrlich designates it as uniceptor. 



The foregoing explains only one type of immunity, 

 namely, antitoxic immunity. Another type remains, anti- 

 bacterial immunity, in the explanation of which Ehrlich 



