Effect of antigen concentration and dilution of whole blood on the IFN-γ response to malaria antigens. IFN-γ (pg/mL) was assayed in supernatants of cells from 3 malaria-exposed (donor nos. 1, ▿; 3, □; and 6, ○) and 4 malaria-nonexposed (donor nos. 2, ; 4, ▪; 5, ▴; and 7, •) donors incubated for 6 days with 0.5, 5.0, or 50 × 10⁵/mL iRBCs (A,C,E), or equivalent amounts of PfSL (B,D,F). PBMC assays (A-B), whole blood diluted 1:10 (C-D), and whole blood diluted 1:3 (E-F) from the same samples were compared. IFN-γ concentrations in control cultures with uninfected RBCs were always below the LDD of the assay (125 pg/mL) and are not shown. Each donor is represented by a different symbol. The dotted line indicates the LLD for the IFN-γ ELISA (125 pg/mL). We therefore made a direct side-by-side comparison between PBMC and WBA for 5 donors.