The Immunization of Rabbits with Soluble Type-specific Substances Derived from Pneumococci Grown In Vivo

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Abstract
Saline extracts were made of lesions secured from rabbits infected intradermally with pneumococcus type I or II. Extract I or extract II obtained when the infecting organism was pneumococcus type I or n, respectively induced a type-specific immunity in rabbits. A tryptic-digest of extract II elicited active immunity in rabbits against intradermal infection with pneumococcus type II. The tryptic-digested material (0.0001 ml) actively immunized mice against 100,000 LD of pneumococcus type II whereas a larger dose (0.1 ml) failed. Extract II and the tryptic-digested material evoked in rabbits the production of mouse-protective antibodies of a type-specific nature. Type-6pecific precipitins or agglutinins were not demonstrable in the mouse-protective serums. The activity of mouse-protective serum resided in the [gamma] 2-globulin fraction. Discussed are experimental results that indicate the immunizing antigen in the extracts I and II may have been the type-specific polysaccharide. Previous failure of investigators to immunize rabbits with soluble extracts of pneumococci was correlated with their use of large doses resulting in "immunological paralysis".