INFECTION AND IMMUNITY. 557 



ting subject in medicine, and none of broader biological 

 significance than that involving this riddle of immunity. 

 For a quarter of a century it has attracted the attention 

 of the most brilliant investigators in medicine and its 

 allied fields, and, though much light has been shed 

 upon it as a result of investigations of recent years, it 

 is not yet fully elucidated. 



It is obviously inadvisable in a work of this character 

 to follow in detail the manifold lines of investigation 

 aimed to clear up this matter. We sliall content our- 

 selves, therefore, with a statement of the significant 

 results and such discussion of them as may be neces- 

 sary to indicate their bearings upon the problem. 

 Knowing as we now do that infection is at bottom a 

 matter of intoxication, and believing, as we are led to 

 do by Ehrlich and his pupils, that intoxication is to be 

 interpreted as a destructive union, in the chemical sense, 

 between the poisons on the one hand and cells or parts of 

 cells for which they have an affinity on the other, natural 

 resistance or immunity from one or another type of 

 infective organism may be interpreted in several ways, 

 namely that the naturally immune animal is by nature 

 devoid of those cells or parts of cells with which the 

 poison of the infective organism from which it is immune 

 has a specific combining affinity, or that the animal is 

 by nature endowed with cells, parts of cells or products 

 of cell life that serve as antidotes for the poison of 

 the infective organism in question ; or, again, that cer- 

 tain cells of the immune animal have the power to 

 actually destroy the infective organism when it gains 

 access to the body, thereby not only preventing its 

 growth and multiplication, but simultaneously rendering 



