ADJUSTMENT TO INFECTIOUS DISEASE 4I5 



precipitating horse serum. By "specificity" in such a case we 

 imply that the serum of an individual so treated precipitates 

 horse serum only, and not, to any extent, any other protein. 

 Since the precipitating property appears to depend upon 

 some newly produced cellular product, it has been assumed 

 that the blood contains a substance absent from the animal 

 originally, conveniently spoken of as an "antibody," in 

 this case "precipitin." 



That there is an actual substance in the serum upon 

 which these reactions depend and that they are not merely 

 the consequences of a change of state is made apparent 

 by the fact that the antigenic substances absorb out of 

 the serum their individual reaction bodies. Thus, if an 

 agglutinating or a precipitating serum is mixed with the 

 bacteria or other antigen upon which this serum exerts its 

 action, the supernatant fluid of such a mixture will be 

 deprived of the capacity to produce the particular eff"ect, 

 and this power can be shown to have been transferred to 

 the precipitate. For to some extent the precipitate can be 

 washed, its unit combinations partially dissociated and the 

 so-called "antibodies" recovered. 



It is this property possessed by substances of a given 

 chemical and physical structure to arouse a specific response 

 on the part of the tissue cells which constitutes the basic fact 

 of immunology. For the bacterial body and many of the 

 bacterial products, toxic or otherwise, belong to this class of 

 antigenic substances, and an infection is therefore nothing 

 more than the entrance of an antigen into the physiological 

 interior of the body, diff'ering from a similar penetration of 

 undigested egg white, milk or any other protein chiefly in 

 that, in the case of bacteria, the antigen is a living cell which 

 can multiply at the expense of the host and often possesses 

 general and specific toxic properties, together with selective 

 powers of localizing or penetrating particular organs or 

 tissues of the host. These diff"erences from ordinary antigens 

 and the fact that, in regard to these variables, no two species 

 of bacteria are entirely alike, has of course necessitated the 

 assembhng of a formidable volume of precise information, a 

 good deal of which is of practical value in diagnosis and 

 treatment. The science of immunology, therefore, is one 



