Chapter p 



The Chemical Nature of Organismal 



Differentials 



In the preceding chapters we have analyzed by means of tissue reactions 

 the individuality and species differentials, as well as organismal differ- 

 entials in general. Immune reactions made it possible to analyze still 

 further the species differentials and the differentials of genera, orders and 

 classes of animals, and even of plants; but, individuality differentials were 

 accessible to serological tests only in a very restricted way. Immune reactions 

 can be used in the study of all those differentials which are able to function 

 as antigens. This includes in addition to the organismal differentials, organ 

 differentials, the heterogenetic differentials of various kinds, and the blood- 

 group differentials, as well as specific antigens present in certain micro- 

 organisms and metazoic cells. 



In the majority of these cases we have to deal with substances which have 

 not yet been isolated chemically, but which can be recognized and differentiated 

 from one another by the tissue and serological reactions which they induce. As 

 to the chemical nature of these substances, our knowledge is therefore very 

 limited. However, there can be little doubt that the organismal differentials are 

 proteins ; this is indicated by their great sensitiveness to heat and to the 

 action of substances which are known to denature proteins. As to the organ, 

 heterogenetic and blood-group differentials, proteins may also enter into their 

 constitution, but they may still retain to a certain degree at least their 

 characteristics as antigens under conditions in which proteins are denatured. 

 Therefore other groups than proteins form part of these antigens. They 

 may be conjugated proteins, combinations of proteins, acting as carriers, and 

 of complex carbohydrates, lipids, or simpler organic substances acting as 

 haptens. The combinations with simpler substances are of significance es- 

 pecially in the state of hypersensitiveness. But even simple inorganic and 

 organic substances as such, seem to be able to induce idiosyncrasy in certain 

 individuals predisposed to this condition, although there is the possibility that 

 even in this case they become effective only in combination with proteins. In 

 all these instances it has been shown that as a rule the antigenic function 

 proper, that is, the production of immune substances or antibodies, requires 

 the combination of these non-protein substances with proteins; but if the 

 antibodies have once been formed, they may interact in a specific manner 

 also with the non-protein material functioning as haptens. However, it has 

 been proven more recently by Heidelberger that in pneumococci, type-specific 

 and species-specific complex carbohydrates are present, which may act as 

 antigens and call forth the production of antibodies without having previously 

 entered into combination with proteins. Specific carbohydrates have been iso- 



565 



