The Collaborative International Dictionary of English v.0.48:

Side-chain theory \Side"-chain` the`o*ry\ (Physiol. Chem.)
   A theory proposed by P. Ehrlich as a chemical explanation of
   immunity phenomena. In brief outline it is as follows: Animal
   cells and bacteria are complex aggregations of molecules,
   which are themselves complex. Complex molecules react with
   one another through certain of their side chains, but only
   when these side chains have a definite correspondence in
   structure (this accounts for the specific action of
   antitoxins).
   [Webster 1913 Suppl.]