Plasma spiking of increasing concentrations of IFN-γ demonstrates high-affinity IFN-γ–binding and neutralizing activity. (A) Recombinant human IFN-γ was added to patient and control plasma at various concentrations and plasma IFN-γ concentrations were determined by ELISA. (B) IFN-γ–binding activity does neutralize IFN-γ activity. PBMCs from a healthy proband were incubated with patient plasma or control plasma with or without recombinant human IFN-γ for 20 hours followed by stimulation with LPS for 4 hours. The supernatants were analyzed for TNF-α and monocytic HLA-DR expression was measured by flow cytometry. Data are shown as ratio of IFN-γ plus LPS/LPS alone.