Ehrlich's side-chain theory, artwork. 1900 diagram by the German immunologist Paul Ehrlich (1854-1915) to explain his side-chain theory of immune response. This theory suggested that the chemical structure of cells was responsible for the action of immune recognition. He postulated that living cells had side-chains that could link with a particular toxin 'like a key in a lock', that under threat a cell grew additional side-chains to bind the toxin, and that these additional side-chains broke off to become the antibodies that circulate through the body.