164 MACROMOLECULAR COMPLEXES 



patches are involved in the reaction between the two immunolog- 

 ically identical proteins and the specific antibody species participat- 

 ing in the precipitate. But the antigenic patches represent only 

 very small areas on the antigen molecule relative to the entire mole- 

 cule (Haurowitz, 1956; Kabat, 1957). Therefore, the combination 

 of the antibody with its specific antigenic patch in itself could not 

 be expected to reveal much direct information about the configura- 

 tion of the remainder of the antigen molecule. Assuming that suit- 

 able precautions have been taken against coincident band forma- 

 tion, a very important fact is provided by the fusion of the bands. 

 Presumably, each band develops in the gel medium along the locus 

 characteristic for the equivalence-ratio for the specific antigen-anti- 

 body system in the gel medium. When two bands fuse, therefore, 

 the equivalence-ratios for the reactants involved in the two bands 

 must have been identical. This tells us that, in all probability, the 

 same number of antigenic patches were present on each antigen 

 molecule; for, had this number been different, the equivalence-ratios 

 for each antigen-antibody complex would not have been the same. 

 Consequently, the locus of each band of precipitate would have 

 been different, and they would not have fused. Therefore, gel- 

 diffusion methods do more than inform us that identical antigenic 

 patches occur on the molecules from different sources; they imply 

 that the same number of antigenic patches are present on each 

 molecule and that the spatial configuration of these molecules is 

 sufficiently congruent to yield identical equivalence-ratios. In view 

 of the previous discussion, it would appear that gel-diffusion tech- 

 niques present us with the most critical criterion currently available 

 for establishing an identity or non-identity between two proteins. 



Very recent evidence (Feinberg and Grayson, 1959) provides 

 some latitude for doubt about an identity between two proteins 

 based solely on the fusion of bands of precipitate. It revealed that 

 grass-pollen antigens cross-reacting with two different antisera could 

 yield band patterns in conventional Ouchterlony gel-diffusion tech- 

 niques suggesting an identity between two antigens, while a slight 

 modification of this technique indicated that only a similarity existed 

 rather than an identity. Any conclusions concerning an identity 

 between two proteins, based on the fusion of bands, must be re- 

 stricted within the limitations, not completely understood, inherent 

 in the procedure itself. The term "identity" will be used throughout 

 the remainder of the paper with this in mind. 



