INFECTION AND IMMUNITY. 437 



2. That when infection occurs it may be explained 

 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 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 auimal ; or the viru- 

 lent bacteria may be so treated that they are no longer 

 of full virulence, and when introduced into the body 

 of the auimal will produce poisons of a much less 

 vigorous nature than would otherwise be the case. 



4. That immunity following the introduction of 

 bacterial 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. 



5. That this protecting proteid that is generated by 

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

 nistic 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. 



6. That in the serum of the normal circulating blood 

 of many animals there exists a substance that is capable, 

 outside of the body, of rendering inert bacteria that, if 

 introduced into the body of the auimal, would prove 

 infective. 



