bonds of the receptor atom groups and the corresponding bonds of the external groups being so related structurally that they dovetail, or in Emil Fischer's well-known analogy, fit each other as lock and key. The extraneous substance is thus in Ehrlich's own terminology "fixed," anchored or bound by the special chemoreceptor groups or the products they throw out. Now what we know of the dynamics of cellular pathology is summed up in Carl Weigert's generalization that the amount of repair of an injured or diseased tissue is usually in excess of what is required, provided the original injury is not too great. When a cell is attacked by a toxin or poison the receptor atom groups will, unless immediately overwhelmed, gradually acquire the power of throwing into the blood detached portions or products of themselves—the antitoxins—and these new side-chains float in the neighborhood of the cell, like so many battleships to protect it from injury. Ehrlich compares these antibodies to lightning rods which draw away the destructive elements to themselves. When administered as therapeutic injections he compares them to "charmed bullets which strike only those objects for whose destruction they have been produced." This is the gist of Ehrlich's theory of immunity and its central idea of a special affinity between drugs and tissues has been the center of gravity of Ehrlich's life work. The unusual terminology which he employs—the various toxophores, toxoids, toxones, amboceptors, haptines, etc.—are simply so many tags or labels affixed to designate complex protoplasmic products of unknown composition with the action of which he has become familiar through long process of experiment. The importance of these imaginative concepts, Ehrlich insists, is in their "heuristic value." How great this heuristic value is, how it has served as an inductive principle in seeking the essentials through the labyrinth of accidentals, is seen in such a triumph of synthesis as the Wassermann method for the serodiagnosis of specific infections. Concerning this discovery Wassermann says that although the side-chain theory was for years an apple of discord among bacteriologists, a thing to look upon askance, yet he could never have hit upon so special a test without Ehrlich's imaginative picture of the mechanism of disease as a guide in his experiments. Protected by parallel control tests with known syphilitic and non-syphilitic bloods, this mode of diagnosis is practically infallible, even in ataxia and paresis. As an important step forward, it changed the whole aspect of those puzzling cases of immunity from syphilis which are known as Colles's law and Profeta's law. By Colles's law a syphilitic
- Die Seitenkettentheorie, der jahrelange Zankapfel im bakteriologischen Lager, hat besonders dazu geführt, Ehrlich in manchem medizinischen Kreisen als 'Theoretiker' auf dem Immunitätsgebiete zu betrachten, und den 'praktischen' Wert der Seitenkettentheorie über die Achsel anzusehen. Demgegenüber kann der Schreiber dieser Zeilen nur sagen dass man ohne die Lehren Ehrlichs beispielweise niemals die Serodiagnostik der Syphilis hätte finden können," A. Wassermann, München. med. Wohnschr., 1909, LVI., 247.