INFECTION AND IMMUNITY. 481 



either by the excess of vigor of the bacterial products 

 over the antidotal or protective proteids produced by 

 the tissues, or to some cause that has interfered with the 

 normal activity and production of these bodies. 



3. That in the serum of the normal circulating blood 

 of many animals there exists a substance that is cap- 

 able, outside of the body, of rendering inert bacteria 

 that, if introduced into the body of the animal, would 

 prove infective. 



4. That immunity is most frequently seen to follow 

 the introduction into the body of the products of growth 

 of bacteria that in some way or other have been mod- 

 ified. This modification may be artificially produced 

 in the products themselves of virulent organisms, and 

 then introduced into the tissues of the animal ; or the 

 virulent bacteria may be so treated that they are no 

 longer of full virulence, and when introduced into 

 the body of the animal will produce poisons of a 

 much less vigorous nature than would otherwise be 

 the case. 



5. That immunity following the introduction of bac- 

 terial products into the tissues is not in all cases the 

 result of the permanent presence of these substances, 

 per se, in the tissues, or of a tolerance acquired by the 

 tissues to them ; but is probably, in certain instances, 

 due to the formation in the tissues of another body that 

 acts as a protecting antidote to the poisonous products 

 of invading organisms. 



6. That this protecting proteid which is generated by 

 the cells of the tissues need not of necessity be antag- 

 onistic to the life of the invading organisms themselves, 

 but in some cases must be looked upon more as an anti- 

 dote to their poisonous products. 



