The following modifications of the antigen (pneumococcus hemotoxin) were studied: (1) the hemolytically active (reduced) substance; (2) the hemolytically inactive, reversible oxidation product; (3) the inactive irreversible products formed by treatment With high concentrations of H2O2; (4) the inactive products formed by heat. The antibody-invoking property of the reversibly oxidized form seemed to be identical with that of the original, hemolytically active or reduced form; neither of the other two hemolytically inactive products invoked antibody production. The same modifications of the antigen which exhibited the antibody-invoking property in vivo possessed the antibody-combining property in vitro; and the modifications which lacked the one property also lacked the other. Evidence is presented that the groups of the hemotoxin molecule in which the true antigenic properties are resident are not necessarily altered by processes which inactivate the groupings responsible for the toxic (hemolytic) action of the original antigen. The lack of antigenic properties on the part of the other two hemolytically inactive modifications is evidence that the treatment employed to alter the toxic property of the molecule must be properly chosen to avoid profound changes which affect the antigenically effective groupings. From an immunological point of view, the reversibility of the antigenically effective oxidation product of pneumococcus hemotoxin is important as an index that the loss of toxicity (hemolysis) was accomplished without a profound change in the molecule.

The theoretical significance of the antigenicity of non-toxic modifications of toxic antigens is discussed.

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