The occurrence and nature of passive protective antibody in 100 samples of human serum was investigated in mice challenged with strains of capsular types A (Smith diffuse strain) and B (strain NS58D). Sixty of the sera passively protected mice against the capsular type-A strain, three against type B, and one against both types. Rabbit antisera against human IgG, IgA and IgM could remove the protective activity from a human serum of high potency, and the activity was also sensitive to 2-mercaptoethanol. Absorption with Smith surface antigen removed protective activity and reduced the concentration of IgG 7-fold, IgA 2.7 fold and of IgM 3-fold more than in a non-protective serum. Consequently, the protective activity of human serum is believed to be associated with antibodies to the capsular antigen in the three immunoglobulin classes.


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