Using ragweed hay fever as the representative of a certain type of allergy we have made studies to determine if possible the mechanism of the protection afforded by specific injections thus far established only by clinical observation.

1. Blood transfusions and serum injections from clinically immune, treated patients stopped the clinical reaction in untreated patients, thus indicating a transferable immunity.

2. The amount of skin sensitizing antibody in the serum was found to be practically unchanged by specific injections.

3. Injection of allergen-antibody mixtures into normal skin showed an immediate (1 hour) reaction when sites were made if serum of untreated cases (Serum A) was used but none or slight reaction if serum of treated cases (Serum P) was used.

4. When sites made with allergen-antibody mixtures were tested after 48 hours, reactions were absent with Serum A mixtures if enough allergen had been used, but were positive with mixtures of Serum P even though a much stronger allergen was contained in the mixture.

5. The primary inhibition of reactions with mixtures including Serum P was not due to antihistamine effect nor to binding of skin sensitizing antibody nor to binding or lysis of allergen.

6. The inhibiting antibody appears to be specific.

7. These serological studies supported by transfusion experiments have been interpreted by us as showing the development under treatment of a peculiar blocking or inhibiting type of immune antibody that prevented the action of allergen on the sensitizing antibody and hence showed in the type of human allergy under consideration (hay fever) the coexistence of sensitization and immunity.

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