162 IMMUNITY. THEORIES OF IMMUNITY 



Specificity of Antibodies. Antibodies are usually specific for their 

 antigen, and it is upon this general law that the reactions of immunity are 

 based. It should be remembered, however, that not all antibodies are 

 protective; the agglutinins, for instance, apparently do not injure their 

 antigen. On the other hand, an animal may enjoy an immunity with- 

 out demonstrating the presence of any antibody in the body-fluids, and 

 another animal may show antibodies generally considered as possessing 

 protective powers, as, for example, the bacteriolysins, without neces- 

 sarily being immune. 



Upon what does the specificity of antibodies and immunologic 

 reactions depend? Specificity was at first believed to depend solely 

 upon some peculiar biologic relationship of the antigens, for it was found 

 comparatively easy to differentiate the serum of animals of dissimilar 

 nature by means of the precipitin and other reactions, and, as serum pro- 

 teins, which seemed to be quite similar chemically, but which were 

 obtained from unrelated species, were sharply differentiated by the 

 biologic reactions, it was considered that the specificity must be depend- 

 ent upon some principle quite apart from the ordinary chemical sub- 

 stances. 



With the use of proteins other than serums, and especially when more 

 or less purified proteins were employed, it has been quite firmly estab- 

 lished that specificity depends upon chemical composition, and that 

 differences in species, as exhibited by their biologic reactions, depend upon 

 distinct differences in the chemistry of their proteins (Wells) . 



Pick and his coworkers have shown that two kinds of specificity 

 exist in each protein molecule: (1) One of these is easily changed by 

 various physical agents, such as heat, cold, and partial coagulation. 

 When an antigen is altered by heat, it produces an antibody that reacts 

 best with the heated antigen; heating does not, however, destroy the 

 characteristics of the antigen of this species, as its antibody will not react 

 with the heated antigen of another species. (2) The second alteration 

 involves a profound chemical change of the antigen, whereby it is so 

 altered that it loses the characteristics peculiar to the species, and pro- 

 duces an antibody that will react with the altered antigen, but not with 

 the unaltered antigen, even from the same animal. For example, it is 

 possible so to alter the serum protein of a rabbit by treatment with 

 nitric acid that the nitroprotein injected back into the same rabbit will 

 produce an antibody specific for the nitroprotein, but which does not 

 react with the unchanged serum protein. These changes are apparently 

 closely related to the aromatic radicals of the protein antigen, for they 



