NATURAL RESISTANCE AND FATIGUE 145 



the circulation of higher animals, were capable of 

 eliciting the production of specific antibodies. The 

 latter by one means or another afford the host some 

 degree of protection against the invading parasites. 

 In general, the antibodies which are formed in re- 

 sponse to an infection are specific towards the species 

 of microorganism which caused their formation. 

 Thus, it is common knowledge that an attack of mea- 

 sles during childhood usually renders an individual 

 immune to further infection by the causative organ- 

 ism; yet no immunity towards other infectious dis- 

 eases results. In this instance the antibodies formed 

 appear to last throughout the remainder of the host's 

 life. If one wishes, this may be regarded as an adap- 

 tation of the host to a new environment — i.e., to one 

 wherein the chances of infection with measles is ever 

 present. 



In many instances, antibodies produced during the 

 course of disease are either negligible in amount or 

 rapidly disappear from the body of the patient follow- 

 ing his recovery. Witness, for example, the all too 

 frequent recurrence of colds, grippe, and so on, in 

 some individuals. Despite the fact that in such infec- 

 tions specific antibodies are not demonstrable, we 

 nevertheless find many individuals who seldom are so 

 afflicted even although they have been exposed to the 

 same predisposing causes as their less fortunate broth- 

 ers. Classical immunology ofifers no explanation other 

 than natural resistance or natural immunity for this 

 commonly observed fact. While such terms may suf- 

 fice to indicate the observed differences, they denote 

 no definite causative factor. 



In order to effect a satisfactory explanation of such 



