Two reaginic sera from atopic patients were fractionated by chromatography on DEAE cellulose columns and by gel filtration through Sephadex G-200 columns. The skin-sensitizing activity in the chromatographic fractions did not parallel the concentration of either γA- or γG-globulin. Gamma G and γA-globulins in the reagin-containing fractions were precipitated with rabbit antibodies which are specific for either γG- or γA-globulin. Essentially all skin-sensitizing activity in the fractions remained in the supernatant after the precipitation of both immunoglobulins. The reaginic activity in the supernatant was coprecipitated with γG-globulin-anti-light chain antibody complexes. These results indicated that the skin-sensitizing antibody was associated with neither γG- nor γA-globulin but with a unique protein having light chain(s) of immunoglobulins. It was also found that 0.001 to 0.002 μg N of this protein was sufficient for the passive sensitization of normal human skin for positive P-K reactions. The possible carrier of reaginic activity was discussed.
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