Abstract
In connection with our studies of tissue hypersensitiveness following repeated injections of protein in rabbits, it seemed of interest to obtain a picture of antitoxin immunity in the same animal that is sensitized to some protein. A quantitative method for measuring skin immunity to diphtheria toxin, based on the Schick test, and for measuring the amount of antitoxin present in the serum of the immune animal, following intracutaneous injections of toxoid has been described. 1 Briefly, the least quantity of diphtheria toxin, in a 0.1 cc. volume, that is capable of producing an erythematous response following an intracutaneous injection in a normal rabbit is determined. This quantity usually ranges from 0.000003 MLD to 0.0000003 MLD. A given amount, such as 0.5 cc. of toxoid is then injected intracutaneously in fractional doses of 0.25 cc. in the same rabbit. After about 3 weeks, it will be found that the least quantity of diphtheria toxin that will produce erythema following intracutaneous injections of 0.1 cc. amounts, is considerably higher than 0.000003 MLD. With an additional injection of toxoid, the amount of toxin necessary to produce an erythematous response might be 0.1 or 1 MLD. With continued injections of toxoid, a skin response may not be obtained until 5 or 10 MLD of toxin are injected intracutaneously.
Parallel with this picture of the development of skin immunity to the toxin is the increase of antitoxin in the blood serum of the immunized animal, also measured by intracutaneous means, and determined as follows: A small dose of diphtheria toxin is chosen that invariably produces a definite inflammatory response in the skin of a normal rabbit when given intracutaneously in a 0.1 cc. amount.
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