1. Equine anti-ovalbumin and equine antihemocyanin, both capable of fixing guinea-pig complement, did not sensitize guinea pigs passively, or did so but feebly.

  2. Equine antihemocyanin did not confer passive anaphylaxis in the rabbit so that fatal shock occurred.

  3. Rabbits' blood was not passively sensitized in vitro by either equine or rabbit-antihemocyanin.

    • An increase of histamine was not found in the blood of guinea pigs sensitized with equine antihemocyanin.

    • An increase of histamine was found in the blood of some guinea pigs sensitized with rabbit-antihemocyanin.

    • The blood of some guinea pigs sensitized with rabbit-antihemocyanin contained a skin-blanching substance.

  4. Guinea-pig antihemocyanin caused a positive Prausnitz-Küstner reaction in the guinea pig's skin but not in the human skin.

  5. Digested equine antihemocyanin evoked the Arthus reaction in rabbits that had been immunized with this material. The Arthus reaction to normal horse-serum was more marked than the reaction to digested material.

  6. The probable and actual mathematical relationships between the antibody in the sensitizing dose and the antigen in the shocking dose have been discussed, and, also, the bearing of such relationships upon the problem of passively sensitizing guinea pigs with equine antisera.

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