A distinction was formerly made between “organized” and “unorganized ferments.” The former term was applied to the minute living organisms, bacteria, yeasts, molds, etc., which bring about characteristic fermentative changes, while the latter term was restricted to enzymes as just described. Since investigation has shown that the changes ascribed to the “organized ferments” are really due to their enzymes, and that enzymes are probably formed by all living cells, the distinction is scarcely necessary at present. PRODUCTION OF TOXINS. The injurious effects of pathogenic bacteria are due in large part to the action of these substances, which in many respects bear a close relationship to enzymes. The chemical composition is unknown since no toxin has been prepared “pure” as yet. It was formerly thought that they were protein in character, but very pure toxins have been prepared which failed to show the characteristic protein reactions. It is well established that they are complex substances, of rather large molecule and are precipitated by many of the reagents which precipitate proteins. Toxins will be further discussed in Chapter XXVII. It will be sufficient at this point to enumerate their chief peculiarities in order to show their marked resemblance to enzymes. 1. Toxins are _dead_ organic chemical substances. 2. They are always produced by _living_ cells. 3. They are active poisons in _very small quantities_.[17] 4. Their action is specific in that each toxin acts on a particular kind of cell. The fact that a so-called toxin acts on several different kinds of cells, possibly indicates a mixture of several toxins, or action on the _same substance_ in the cells. 5. Toxins are very sensitive to the action of injurious agencies such as heat, light, etc., and in about the same measure that enzymes are, though as a rule they are somewhat more sensitive or “labile.” 6. Toxins apparently have maxima, optima, and minima of temperature for their action, as shown by the destructive effect of heat and by the fact that a frog injected with tetanus toxin and kept at 20° shows no indication of poison, but if the temperature is raised to 37°, symptoms of poisoning are soon apparent. Cold, however, does not destroy a toxin. 7. When properly introduced into the tissues of animals they cause the body cells to form antitoxins (Chapter XXVII) which are capable of preventing the action of the toxin in question. 8. _The determining test for a toxin is its action on a living cell._ It is true that enzymes are toxic, as are also various foreign proteins, when injected into an animal, but in much larger doses than are toxins.