According to Ehrlich, the following are the characteristics of toxins:

1. They are extremely easily destroyed (labile) substances which occur as secretion products of vegetable or of animal organisms.

2. Their chemical nature is unknown. The impossibility of obtaining them in a pure form and their great lability render them insusceptible to ordinary chemical analysis.

3. An analysis of a toxin may be reached at present only through the medium of animal experiment.

4. The introduction of toxins into the tissues causes the formation of an antitoxin with the production of immunity. It has not been possible to obtain antitoxins for inorganic poisons, as the alkaloids.

5. In contradistinction to well-defined chemical poisons, the action of toxins is characterized by a latent or incubation period. The incubation period may be shortened experimentally by the injection of large quantities of toxin, but it cannot be eliminated entirely. Snake-venom, however, seems to act without an incubation period, but it is still to be classed with toxins because of its power to cause the formation of antitoxin.

6. The facts make it necessary to assume as a condition for the poisonous action of toxins a specific union with the protoplasm of the cells in certain organs. The affinity of other poisons, as alkaloids, for tissues depends not upon specific chemical union, but on some such process as solid solution or loose salt formation.

The true toxins cause the physiologic and pathologic disturbances as a result of their solubility and the ease with which they can diffuse throughout the animal juices. The two chief toxin producers are the organisms of tetanus and diphtheria. A few of the tetanus bacilli may cause no local disturbance, yet may bring about the death of the individual. Toxins produce specific symptoms; consequently, it may be assumed that they have special selective affinities for certain tissues, and produce their symptoms in consequence of such affinity. This can be shown experimentally: a mixture of guinea-pig brain and tetanus toxin will prove harmless, although there may be present several times the fatal dose of the toxin. Other toxins, instead of being specific for the motor nerve-cells, may act upon the red blood-cells, or upon the leukocytes, or upon the cells of the respiratory centers, for instance.

Endotoxins are insoluble substances not secreted by the living organism, but set free only after the death and disintegration of the parasites. They are not as specific in their action as the true toxins, but their injection into suitable animals gives rise to the production of antitoxins which are capable of neutralizing the endotoxin employed. The toxic effect rapidly diminishes on keeping, and is seriously impaired by exposure to higher temperatures - 55° to 6o° C. Many symptoms of disease may be due to the breaking down of the organism with the liberation of these bodies.

Bacterial proteins constitute the main mass of the organism. They differ from the toxins and endotoxins in not conforming to the characteristics of either of the two. Their effect is essentially pyogenic, the formation of pus, and is one common to most, if not all, bacteria. In some animals the pyogenic action does not manifest itself, because death results too early, but in more resistant individuals it can be shown. These proteins in themselves are not markedly dangerous, but they have gained in importance since it has been demonstrated that the introduction of foreign albumins leads not to increased resistance (immunity) against such proteins, but to hyper-sensitiveness (anaphylaxis). Consequently, a subsequent injection after a certain interval of time may produce serious symptoms or death.

Ptomains are nitrogenous compounds of basic nature and alkaloid-like properties, formed from animal matter in consequence of bacterial decomposition. Their formation is only possible when special food stuffs are directly available, while toxin production is, within certain limits, independent of the food supply, and represents a specific function on the part of the micro-organisms in question.