When, for any reason whatever, this phagocytic action is impeded, the resistance of the organism to pathogenic infection ceases to be effective, and the organism may therefore be invaded by the microbe. Numerous causes may contribute to impede this action.
We have seen that the second means of defense possessed by the organism resides in the action of special products, true defensive secretions, possessing an activity contrary to that of the toxins, and which are secreted by the cells of the organism under the influence of the vaccins.
This is a property common to every organism, and which is observed even in non-vaccinated subjects, although in this case the secretion forms with great difficulty and in small quantity.
When an organism subjected to the toxic action of a bacterial infection does not succumb to the intoxication, it emerges from the test gifted with a new property, which may be augmented by habituation, and which borders on immunity.
At first we were content to vaccinate small animals in the laboratory, but in proportion as the discoveries in this domain extended, and there developed a need for large quantities of antitoxins, recourse was had to the larger animals, particularly horses and cattle. From the moment that large quantities of blood and antitoxic serum were at command, search was made for a means of isolating the antitoxin and determining its properties.
Experiments so far made have shown that the antitoxins are substances of an albuminoid nature, of unknown composition, and which are very closely united to the albuminoid substances of the serum. It must be observed, however, that Behring and Knorr oppose the assertion regarding the albuminoid nature of tetanic antitoxin, but their reasons for this do not appear to be well founded.
In general, these antitoxins are precipitable with the globulins, and possess quite considerable powers of resistance towards physical and chemical agents. Thus they are destroyed only at a temperature above 60-65° C. Kept in the dry state, in the residue of evaporated serum, and away from the light and all oxidizing action, it is possible to preserve their activity for a very long time.
They are essentially humoral substances; they are found in the blood of vaccinated animals, from which may be obtained antitoxic serums with a specific but transient immunity; and they are also found in the plasmas of the lymph and exudates, in aqueous tumors, and in the milk. They are seldom found in the cells.
Mode of Action.—Frequent attention has been paid to the mode of action of the antitoxins upon the toxins, a phenomenon of great importance in relation to the phenomenon of immunity acquired against the toxins. At the beginning of our knowledge on this subject, the idea of a destruction of the toxin immediately suggested itself, and was advanced by von Behring.[43] According to this scientist the antibody inhibits the morbigenic action of the toxin by neutralizing the toxin, combining with the latter to form a compound of a chemical nature which is devoid of toxicity and without action on the organism. According to this theory, the influence of the antitoxin on the toxin is direct, and does not require the intervention of the living cellular protoplasm. Such was also the belief of Prof. Ehrlich.[44]