Passive protection by human serum in mice infected with encapsulated Staphylococcus aureus.

The occurrence and nature of passive protective antibody in 100 samples of human serum was investigated in mice challenged with strains of Staphylococcus aureus 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 S. aureus capsular antigen in the three immunoglobulin classes.