The data presented in this paper clearly indicate that the isolated carbohydrate and nucleoprotein constituents of Pneumococcus differ both serologically and antigenically one from the other. Moreover, each of these fractions of the cell separately exhibits immunological properties distinct from those manifested by the whole organism of which they form a part.

The carbohydrate is a protein-free polysaccharide and as such is devoid of the property of stimulating antibodies. Although in the free state, dissociated from other cellular substances, it is non-antigenic, in this form it still retains the property of reacting specifically in anti-pneumococcus serum of the homologous type. Further, this nonprotein constituent is not reactive with antiprotein serum. In other words, neither pneumococcus carbohydrate nor protein as separate antigen gives rise to antibodies with specific affinities for the carbohydrate or so called soluble specific substance of Pneumococcus.

The nucleoprotein of Pneumococcus, on the other hand, is antigenic. Immunization with this cell constituent gives rise to immune serum which precipitates solutions of pneumococcus protein without regard to the type from which it is derived.

The interrelations of the carbohydrate and protein of Pneumococcus as they exist in the intact cell to form the complete antigen, and the interpretation of the differences in the antigenic properties of the whole bacterium as contrasted with those of its component parts are reserved for discussion in a subsequent paper.

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