Antigen-specific cytophilic activity of sheep IgG1 and IgG2 antibodies.

Sheep were immunised with chicken red blood cells in the area drained by the popliteal lymph node and lymph was collected from the stimulated nodes. The immune lymph samples were fractionated into IgG1 and IgG2 antibody fractions; these were then tested by passive direct and passive indirect rosette tests for antigen-specific cytophilic activity on adherent mononuclear cells (macrophages) from non-immunised sheep. Antigen-specific cytophilic activity for homologous macrophages appeared within a week following primary immunisation. Fractionation of the immune lymph showed that antibodies associated with IgG1, but not IgG2, subclass were highly active in promoting binding of the antigen to the macrophages. Pretreatment of the macrophages with uncomplexed IgG1 from non-immunised sheep inhibited binding of cytophilic IgG1 complexed with antigen. The binding of non-immune uncomplexed IgG1 to the macrophages appeared to be very weak since washing of cell monolayers after passive sensitisation with non-immune, uncomplexed IgG1 overcame the inhibition of binding of antibody IgG1 complexed with antigen.