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.
Ehrlich supposed that living cells have side-chains in the same way dyes have side-chains which are related to their coloring properties. These side chains can link with a particular toxin, just as Emil Fisher said enzymes must bind to their receptors "like a key in a lock."
Ehrlich theorized that a cell under threat grew additional side-chains to bind the toxin, and that these additional side chains broke off to become the antibodies that are circulated through the body. It was these antibodies that Ehrlich first described as "magic bullets" in search of toxins.