High-affinity IFN-γ–binding activity is an anti–IFN-γ IgG4 autoantibody. Patient serum (▪) or control serum (□) was added to plates coated either with recombinant human IFN-γ or recombinant human IL-10. Following extensive washing and incubation with POD-labeled antihuman IgG isotype antibodies, the substrate reaction was measured. OD indicates optical density.