Diagram illustrating German immunologist Paul Ehrlich's (1854-1915) lecture "On Immunity with Special Reference to Cell Life," read 22 March 1900. In this lecture, Ehrlich described his "side-chain" theory of the immune system. 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.