332 PROPERTIES OF THE COLLOIDAL CONSTITUENTS 



any case identical with one another. Now as far as our experience ex- 

 tends, all antigenic substances are proteins. All attempts to demonstrate 

 antigenic properties in substances unrelated to proteins have resulted 

 in failure and in particular the investigations of Fitzgerald and Leathes, 

 have shown that the Lipoids are non-antigenic. Yet, as we have seen 

 the individual proteins which may be isolated from the tissue-fluids are 

 identical in widely differing species. 



If, however, the individual proteins which are separable from blood- 

 serum by chemical procedures are present wholly or in part in the 

 unaltered serum in the form of complexes of several proteins united 

 together, we can readily understand how different sera come to contain 

 differing antigens. Two protein-complexes of this type might well 

 be built up out of identical units, and yet differ fundamentally, owing 

 to differences in the combining proportions, and consequently in the 

 mode of linkage of these units. Just as a wide and conceivably infinite 

 variety of proteins may be built up out of differing permutations 

 and combinations of eighteen or nineteen amino-acids, so an infinite 

 variety of protein complexes might be built up by the union in varying 

 proportions and arrangements of the comparatively limited number of 

 different proteins which are individually separable from a tissue-fluid. 



In pursuance of this idea Gay and Robertson and C L. A. Schmidt 

 have investigated the antigenic properties of several compound pro- 

 teins. If compound proteins differ in their biological specificity from 

 their constituents, then a Compound Protein should represent a new 

 Antigen giving rise to antibodies for itself, as distinguished from the 

 antibodies for its constituents. Unfortunately a formidable technical 

 difficulty stands in the way of clearly recognizing the presence of anti- 

 bodies which are specific for the compound protein. This is the diffi- 

 culty which is constituted by the fact that any protein which is capable 

 of being split by hydrolysis into moieties which are still proteins (in 

 the sense that they are antigenic) gives rise on injection into animals 

 to antibodies not only for itself but also for these split-products. 

 Analogously, a compound protein gives rise to antibodies for its con- 

 stituent parts, and it is only possible to distinguish between these, 

 which would appear in the blood of immunized animals after injection 

 of the separate constituents, and any antibodies which may be formed 

 for the compound as a whole, in those doubtless exceptional instances 

 in which the antibody for the compound reacts with a constituent 

 which is not normally antigenic. 



The above-mentioned observers have therefore investigated, from 

 this point of view, certain compound proteins in which one constituent 

 is non-antigenic, such as Protamine Caseinate, of which the protamine 

 constituent is non-antigenic and toxic, while the casein constituent is 

 antigenic and non-toxic, and Globin Caseinate of which the globin con- 

 stituent is toxic and non-antigenic. 



Protamine caseinate displays no antigenic characteristics which 



