=Formation of Antitoxins.=--Ehrlich's theory of side chains, to which reference has already been made, furnishes us with an explanation of the formation of the antitoxins in tumors. Let us suppose that, in the organism, a cell had come into contact only with certain toxic molecules incapable of compromising its life, and that the only result was the immobilization of the receptors which are united with the haptophore groups of the opposing toxins. It is known that, by virtue of a property inherent in all living organisms, during the phenomena of reparation, there is generally an overproduction of the neoformed parts. In the case we here speak of, as the receptors fill an important function in the nutrition of the opposing cellular elements, once they become united with the toxic haptophores, they become incapable of filling their normal function of nutrition. Under these conditions the cells develop so large a quantity of receptors that, filling the cells, and not finding any more room, they spread into the blood and other liquids of the organism. Under these conditions, every new injection of toxin into the organism is absorbed into the blood where it meets with the free receptors which possess great avidity for the haptophore group of its molecule, and the two groups immediately unite, before the haptophore group of the toxin has been able to attack and intoxicate a cellular element. We thus see that the receptors which, when in a free state in tumors, play the rôle of antitoxics or antitoxins, become, within the cellular elements themselves, the vehicle of intoxications. Figuratively speaking, so long as these fixators are attached to the molecule of the living protoplasm they attract the toxin. According to this ingenious conception, the formation of antitoxins is hence absolutely independent of the action of the toxophore elements on the cellular elements, and it suffices that these possess receptors or side chains capable of uniting with the haptophore groups of the toxin. This explains why it has been possible to produce antitoxins from toxins which have lost some of their toxic properties, but which have preserved their property of uniting with antitoxic substances. Ehrlich gives the name _toxoids_ to those modified toxins that have lost their toxophore groups, while the haptophore group, the producer of the immunizing substance, is still preserved intact. According to Metchnikoff's theory, which is very similar, it seems quite possible that the phagocytes, thanks to the facility with which they absorb poisons, occupy an important place as producers of antitoxins. It has not been possible so far to verify this theory in our at present imperfect knowledge regarding this subject. The domain of immunity has, however, made brilliant conquests during these last few years, so that we should not despair of arriving at a definite solution before long.