ADJUSTMENT TO INFECTIOUS DISEASE 419 



The change in cell capacity therefore is the essential fact 

 and the discharge of the reaction substances into the cir- 

 culation purely secondary, depending in some manner 

 upon the chemical and physical properties of the particular 

 antigen. 



The most perfect antigenic substances are the proteins 

 and it is from the study of these that most of our knowledge 

 is derived. It would appear that the antigenic function may 

 be in some manner related to non-difFusibiHty, since the 

 nature and molecular size of antigenic substances seem to 

 imply reaction with the cell surfaces. Teleologically regarded, 

 this may mean that substances that can penetrate into the 

 cells and undergo intracellular digestion may not require 

 the development of a special antibody mechanism. The 

 second criterion necessary to the possession of antigenic 

 function seems to be an inability of the healthy body to 

 eliminate these substances promptly by the ordinary 

 means of excretion. Take the case of egg white. This antigen 

 is eliminated by monkeys almost quantitatively within a 

 few hours, little or no antibody is formed and hypersensi- 

 tiveness develops only to a slight degree. And when in 

 animals like rabbits, or even in man, antibodies are studied in 

 response to the injection of horse serum, the horse seru*m 

 may be found circulating for days, disappearing only grad- 

 ually as antibodies begin to form. Thus, the most effective 

 antigens are substances which are not easily eliminated, 

 which are not removed from the circulation with facility and 

 which, presumably on this account, form a slow union with 

 the cells of the body. 



It seems quite clear that the antigenic proteins, besides 

 possessing the essential physical properties mentioned above 

 must undergo a very definite chemical union of some kind 

 with the responding cells, which is, indeed an inevitable 

 conclusion from investigations on the nature of specificity. 



It is in this matter of specificity that the immunological 

 reactions illustrate, more than any other physiological 

 phenomena, the exquisite powers of adjustment of the animal 

 cell. In no other group of biological observations is speci- 

 ficity so finely differential and so manifold. The response of 

 any given species of animal in essential mechanism to the 



