THE DEFENSES OF' THE BODY 267 



tinates Bacillus typhosus and no other organism, that patient 

 in all probability is suffering from typhoid fever. This 

 latter application of the reaction constitutes what is gener- 

 ally known as the Widal reaction. 



Immunity: Historic Sketch. — In the course of our studies 

 aimed to secure light on the mechanism of infection, two 

 phenomena are constantly in evidence, notably — first, 

 that not all individuals are susceptible to infection by all 

 pathogenic bacteria, and next, that an individual who has 

 recovered from infection has undergone a change during 

 the course of the disease that, as a rule, renders him 

 insusceptible to subsequent infection by the same species 

 of bacteria. Individuals in either the one or the other 

 state are said to be immune; in the former to be immune by 

 nature, in the latter to have acquired immunity. 



In its present development there is no more fascinating 

 subject, and none of bjoader 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 cognate fields, and, though 

 much has been learned, it is as yet far from 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 shall content ourselves, therefore, 

 with a statement of the significant results and such discus- 

 sion of them as may be necessary 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 



