Evidence was obtained that rabbit homocytotropic antibody against dinitrophenyl-bovine γ globulin (DNP-BGG) belonged to a unique immunoglobulin other than γG, γA or γM. The skin-sensitizing activity was precipitated with a guinea pig anti-rabbit globulin serum which did not contain any of the antibodies specific for the three immunoglobulins and for light chains of immunoglobulins. The guinea pig antiserum gave a β precipitin band with a rabbit serum fraction which contained high skin sensitizing activity and the precipitin band combined 131I-labeled DNP-BGG. When rabbit antisera having the homocytotropic antibody were fractionated by chromatography on a DEAE cellulose column, gel filtration, sucrose density gradient ultracentrifugation and zone electrophoresis on agarose gel, distribution of the homocytotropic antibody paralleled those of the β globulin antibody detected by radioimmunodiffusion. Light chains o f immunoglobulins were detected in the β globulin, indicating that the protein represents an immunoglobulin. Since the physicochemical properties and biologic function of the β immunoglobulin were similar to human γE, the protein was tentatively designated rabbit γE. Guinea pig antibody against γE-induced an increased permeability of rabbit skin small vessels.
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