IMMUNITY. 387 



be without effect in the animal body. The 

 civet-cat is immune against the bite of a ser- 

 pent and its serum is antitoxic against snake 

 venom, so that the reason for the natural toler- 

 ance of the poison might be thought to be the 

 possession of this quality. The blood of the 

 hedge-hog, however, which is likewise immune 

 against snake venom, contains also, but in much 

 slighter amount, an antidote against the snake 

 venom. Swine, on the other hand, which are 

 also immune bear in their serum no antitoxin 

 against snake venom, and dogs, which do suc- 

 cumb to snake bite, possess in their blood an 

 antitoxin against snake venom. The anti- 

 toxin is therefore sometimes present, some- 

 times absent, sometimes it is of assistance in 

 protecting an animal, sometimes not. Ani- 

 mals immune to tetanus, and likewise animals 

 immune to abrin, succumb to snake venom, in 

 spite of the fact that both tetanus antitoxin 

 and anti-abrin paralyze in vitro much larger 

 quantities of snake venom than would suffice 

 to kill these animals. Diphtheria serum 

 mixed with abrin retards the poisonous effect 

 of the abrin in the animal. 



5. A serum or antitoxin which in vitro does 

 not act upon the poison may yet exert an an- 

 tidotal or curative effect when injected into the 

 body of the animal. Rabies serum does not 



