The serum of an animal rendered immune in this manner contains a diphtheria antitoxin which possesses high power. A guinea-pig which has received an injection of 0.01 Cc. of the antitoxin is perfectly capable of withstanding a lethal dose of 0.5 Cc. of the toxin. The antidiphtheria serum thus obtained, and in almost limitless quantities, from an immunized animal, is capable of saturating the therapeutic diphtheritic toxin, and has to-day taken rank in therapeutics as the most efficacious remedy in diphtheria. Injected in varying doses, it confers a temporary but immediate immunity.
Nevertheless antidiphtheria serum must not be considered as an antidote; and in pathological diphtheria, the more serum is required the later it is used.[113] In certain cases, if employed too late, it may prove ineffective.
The preventive action of the serum is remarkable. In 10 000 inoculated cases Behring and Ehrlich have had but 10 cases of diphtheria, and these were, moreover, of a benign character. The duration of the immunizing action appears to be from three weeks to two months.
This diphtheria antitoxin was first prepared by Guérin and Macé[114] by adding to the antidiphtheria serum a large volume of alcohol, washing the precipitate, and drying it in a vacuum. It is soluble in water, and loses its activity when heated to 65° C. Wassermann[115] has proposed to extract it from the milk of immunized animals, by first coagulating the milk by rennet in the presence of sodium chloride, filtering, and removing the fat from the clear liquid by means of chloroform. After decanting, the clear solution obtained is precipitated by adding to it 30 to 33 per cent. of ammonium sulphate. The precipitate is dried in a vacuum on a polished porcelain slab after having first been strongly expressed. It is then dissolved in water.[116]
Tetanus Toxin.—The fact that the tetanus bacillus never penetrates to the interior of the organism enabled us long ago to foretell that it secretes a very powerful toxin capable of dialyzing and diffusing through the economy. Kuno Faber was the first to fully recognize the fact that the culture bouillon of this bacillus, fully sterilized by filtration through porcelain, possesses an exceedingly high toxicity, and exerts a toxic effect on 50 000 000 times its own weight of living organism. Brieger had previously, however, extracted three ptomaines from the cultures of the bacillus—tetanin, tetanotoxin, and spasmotoxin.[117] In order to obtain a highly active liquid, the same culture medium is inoculated several times in succession, but filtering each time before the new inoculation; the microbes greatly increase in number after each fresh inoculation, and the toxic substance developed by them accumulates.[118]
Experiment has shown that the culture bouillon thus obtained contains two kinds of toxic substances[119]—highly toxic alkaloidal bases (ptomaines, tetanin, tetanotoxin, etc.), and a true toxin, possessing diastatic properties, and of almost incredible toxic power.
This toxin had already been isolated by Kitasato. It is a toxalbumin, and is very sensitive to the action of heat. A temperature of 65° C., maintained for 30 minutes, renders it quite inactive; and it becomes oxidized and is destroyed by the action of the air in the presence of light.
Brieger and Boer,[120] by precipitating with zinc chloride the filtered culture bouillon, obtained a pure, amorphous tetanus toxin, which they also considered as a toxalbumin, and which possesses exceedingly toxic properties.
If a precipitate be caused to form in these toxic solutions, as, for instance, a precipitate of calcium phosphate, this carries down with it all the toxin present in the liquid. 0.0005 Gm. of this precipitate is surely fatal to a guinea-pig.
Dozon and Cournemont have observed that even in doses of 300 to 400 Gm. of the filtered culture liquid, this toxin is not immediately toxic to a horse, but kills the animal only after a period of incubation of at least twenty-four hours. The blood of such an animal, however, is immediately and directly fatal to animals into which it is injected.[121]