Prolonged immunization of rabbits with a sedimented, heat-killed vaccine of Pasteurella boviseptica grown in an infusion broth prepared from rat brain resulted in the production of antisera containing antibodies for the broth as well as for the bacteria. When guinea pigs were prepared by intra-abdominal injection with such antisera and tested 24 hours later by intravenous injection with autoclaved extracts of different organs and tumors of rats, they were found to be passively sensitized, so that severe or fatal anaphylaxis was obtained with extracts of brain and carcinoma 256,—a transplantable tumor of the rat which originated in the mammary gland,—and very slight or negative reactions with extracts of other tissues. The brain antigen was found to be organ-specific but not species-specific. It was present in the white matter of the central nervous system and in sciatic nerve, but was almost completely absent from the brains of fetal and newly born rats and rabbits. It was absent from the brain of the fetal guinea pig but was present very soon after birth. The amount of the specific brain antigen seemed to be dependent upon the length of the period of gestation, the stage of development of the animal at birth, and the degree of myelinization of the central nervous system. The anaphylactogen of brain broth was soluble in water and insoluble in strong ethyl alcohol. It was thermostable in neutral and slightly acid solutions but more or less thermolabile on the alkaline side of neutrality.

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