A. TYLER 



343 



organism, as illustrated in Fig. 1, or if there are similarities in 

 structure, cross reactions are to be expected. Cross-reacting an- 

 tisera are also obtained if the preparation of immunizing antigen 

 contains other active substances as impurities. However, it is not 

 easy to decide in different cases to what extent cross reactions are 

 due to impurities in the original immunizing antigen and to what 

 extent these are due to similarities in determinant groups. Even 

 with highly purified proteins used as antigens, such as the proteins 



antl-A 



anti-X 



antl-B 



antl-X 



anti-A 



antl-Y 



antl-A-antl-X 

 (antl-AX?) 



antl-B- anti-X 

 (antl-BX?) 



antl-A-antl-Y 

 (anti-AY?) 



Fig. 1. Illustration of the assumption that a particular protein may 

 possess two (or more) kinds of determinant groups, one of which (X) 

 may be present also on other proteins of the same species of organism and 

 the other of which (A) may be present on the same type of protein in a 

 different species. The types of antibodies that may be expected upon im- 

 munization are listed below each antigen. See text for discussion in rela- 

 tion to cross reactions. 



of chicken egg-white studied by Cohn, Wetter, and Deutsch 

 (1949), cross-reacting antibodies are obtained. 



Absorption techniques eliminate to some extent certain of these 

 difficulties. However, it is perhaps important to emphasize that 

 in the use of an antiserum as a test reagent, one does not detect 

 a specific protein or other large molecular substance but rather 

 specific determinant groups. These do not necessarily characterize 

 one particular ( chemically defined ) protein, and it is also possible 

 that during embiyonic development they may not remain con- 

 tinuously with the same protein. The term antigen is, therefore, 



