156 INFECTION AND IMMUNITY. 



mersing the animal in cold water, the reduction 

 in body temperature supposedly decreasing the re- 

 sistance. It was stated previously that physical 

 exhaustion, hunger and exposure to cold may also 

 reduce natural resistance. Pestilence and famine 

 often go hand in hand. 

 Relative Similarly, immunity to toxins usually is rela- 



Iiumunity. ,. A -n < < i -. 



tive. As an illustration of natural immunity to 

 toxins, the following table serves a good purpose. 

 The horse is the animal of greatest susceptibility 

 to tetanus toxin. If the minimum fatal amount of 

 one gram of horse weight is taken as a unit, this 

 scale of resistance for some other animals is 

 obtained (Knorr) : 



For 1 gram of guinea-pig weight 2 units are fatal 



For 1 gram of goat weight 4 units are fatal 



For 1 gram of mouse weight 13 units are fatal 



For 1 gram of rabbit weight . . 2,000 units are fatal 



For 1 gram of chicken weight 200,000 units are fatal 



In view of the high immunity of the chicken 

 a g a i ns t tetanus, one may be led to suppose that its 

 serum would contain a large amount of antitoxin, 

 yet experiments show that it possesses practically 

 no tetanus antitoxin. This fact suggests that 

 there is a distinct type of natural immunity 

 which, it is thought, may be independent of both 

 the antibacterial and the antitoxic properties of 

 the body. 



It is now thought that the toxic elements of bac- 



Receptors. ? 



teria are chemical substances (very complex, 

 surely) which are able to injure the tissues, i. e., 

 to cause disease, only by entering into chemical 

 union with substances which the cells contain. 

 Such chemical substances or groups, pertaining 

 to the cells, will be referred to later under the 

 name of cell receptors. Accordingly, if the cells 



