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).

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