REACTIONS OF RABBITS TO INTRACUTANEOUS INJECTIONS OF PNEUMOCOCCI AND THEIR PRODUCTS : II. RESISTANCE TO INFECTION

1. Injection of suspensions of heat-killed pneumococci into the skin of rabbits is followed by an active immunity which is effective against intravenous infection by homologous and heterologous types of Pneumococcus. 2. This form of active immunity may be induced by the injection of S or R strains of Pneumococcus. 3. Intracutaneous immunization with soluble derivatives of Pneumococcus does not induce active immunity to infection. 4. The sera of seventy-nine per cent of the rabbits immunized to Type I Pneumococcus by intracutaneous injections afforded no protection to mice against infection with pneumococci. 5. None of the sera of rabbits intracutaneously immunized to the type-specific Type III (S) pneumococci, to R cells, or to soluble derivatives of Pneumococcus protected white mice against infection. 6. The sera of rabbits immunized first intracutaneously and subsequently intravenously possess a high titre of protective antibodies. 7. It may be concluded that when type-specific pneumococci are injected into the skin they lose the property of stimulating an active immunity of a specific type and of stimulating the production of type-specific antibodies, but they act just as do the degraded or R forms, causing the animals to become resistant to infection with pneumococci of all types without the development of any type-specific antibodies in the serum.