Abstract
In the preceding communication it was shown that the antibody response elicited in rabbits vaccinated intracutaneously with specific type strains of Pneumococcus is characterized by the presence in the serum of antiprotein antibodies in excess, together with a corresponding decrease or complete absence of type specific antibodies. That this change in the order of specific antibodies is related to the mode of inoculation was shown by the fact that the same strain (Type I Pneumococcus), when injected intravenously, invariably stimulates the formation of the dominant type-specific antibodies.
The present paper summarizes the results of a study of the occurrence and nature of the active resistance to infection which develops in rabbits vaccinated intracutaneously with R and S forms of Pneumococcus. Briefly, it may be stated that, following repeated skin inoculation of heat-killed suspensions of type-specific pneumococci or of the R variants, rabbits acquire a considerable degree of active immunity against infection with virulent strains of homologous and heterologous types. For example, rabbits, immunized intracutaneously with Type I may survive the intravenous injection of a dose as large as 0.2 cc. of homologous culture, the virulence of which is such that 0.0000001 cc. kills normal control animals. Similarly, rabbits immunized in the same manner to an avirulent strain of Type III may survive an infecting dose of from 0.5 to 1 cc. of a culture of the homologous type which by animal passage has been rendered so virulent that 0.001 cc. to 0.0001 cc. is sufficient to kill normal rabbits. Furthermore, animals inoculated intracutaneously with Type III become as resistant to subsequent infection with Type I as animals similarly immunized with homologous organisms. Even when an R strain of Pneumococcus derived from Type II is used for injection, rabbits acquire a high degree of resistance against infection with virulent Type I organisms.
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