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IgGa, in contrast to IgE, antibodies mediated the antigen-induced release of histamine from rat peritoneal mast cells without a requirement for a latent period and without the capacity to bind firmly to the target cell. Nonetheless, IgGa anti-DNP antibody interfered with the capacity of rat anti-N. brasiliensis antiserum rich in IgE antibodies to prepare the target cells for histamine release by worm antigen. Further, interaction of IgE antibody-prepared cells with IgGa anti-DNP antibody and DNP-BSA at 0 degrees C so as to achieve sterile activation, or at 30 degrees C to permit histamine release, inactivated such cells as determined by the subsequent failure to release histamine upon challenge with worm antigen. Thus, although IgE and IgGa antibodies are immunochemically distinct homologous immunoglobulins and exhibit different functional characteristics, their interaction at the target cell involves a common receptor and at least one common point in the pathway to the release of pharmacologic agents from the cell.