What is "side-chain theory"

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).

The side-chain theory ( German, Seitenkettentheorie) is a theory proposed by Paul Ehrlich (1854–1915) to explain the immune response in living cells. Ehrlich theorized from very early in his career that chemical structure could be used to explain why the immune response occurred in reaction to infection. He believed that toxins and antitoxins were chemical substances at a time when very little was known about their nature. The theory explains the interaction of antibodies and antigens in the blood, and how antibodies are produced.