Reactions between Certain Strains of Pneumococci and Fc of IgG

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Abstract
A non-immune reaction is known to occur between the Fc region of certain IgG subclasses and cell wall components of staphylococci (protein A) and streptococci. Pneumococci have been studied for similar activity. Seventy-six strains of Diplococcus pneumoniae were evaluated for non-immune reactivity with γ-globulin Fc by testing their ability to agglutinate erythrocytes which were coated with sub-agglutinating quantities of anti-erythrocyte antibody. Thirty (39%) of the strains were reactive. All strains of some pneumococcal serotypes were reactive, whereas all strains of other serotypes were negative. The reactivity appeared to be a property of the polysaccharides since particles coated with purified polysaccharides from reactive serotypes also caused agglutination. When the capsule size of pneumococci was varied by using encapsulated and non-encapsulated mutants, augmentation by mouse passage, or varying cultural conditions, reactivity was greater if the capsule was larger. Inhibition experiments showed the reactivity to be in the Fc fragment, and IgG1 was the only highly reactive immunoglobulin. IgG2 and IgA were probably non-reactive; and IgG3, IgG4, and IgM were clearly non-reactive.

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