The soluble specific substance of Pneumococcus Type I has been chemically isolated from the bacterial cells and from autolyzed cultures as an acetyl polysaccharide.

So far as could be determined by the methods employed, the acetyl polysaccharide in highly purified form absorbs from Type I antipneumococcus serum all demonstrable type-specific precipitins, agglutinins and protective antibodies.

Mice injected intraperitoneally with minute quantities of the acetyl polysaccharide develop active immunity to subsequent infection with Pneumococcus Type I. The immunity thus induced is type-specific. In several instances purpura has been observed in mice following the injection of larger amounts of the acetyl polysaccharide.

Under the experimental conditions of this study, no type-specific precipitins, agglutinins or protective antibodies were demonstrable in the serum of rabbits following repeated intravenous injections of the Type I acetyl polysaccharide. The treated rabbits were not immune to subsequent infection with Pnemnococcus Type I.

The acetyl polysaccharide is readily converted into its deacetylated derivative by treatment with dilute alkali.

The chemical and immunological properties of the deacetylated polysaccharide are identical with those of the soluble specific substance in the chemical form in which it was originally isolated; the deacetylated form of the specific carbohydrate is non-antigenic, does not produce purpura in mice, and only incompletely absorbs the type-specific antibodies from Type I antipneumococcus serum.

The immunological significance of the acetyl polysaccharide and its possible relationship to the specific substances isolated from Pneumococcus Type I by other workers are discussed.

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