Effect of prior infection with dengue virus on expression of interferon-gamma (IFN gamma) in persons with IgG titers ≥ 40 against dengue virus when tested by enzyme-linked immunosorbent assay in Havana, Cuba. Mock, peripheral blood mononuclear cells (PBMC) challenged with uninfected C636 supernatant; D1, PBMC challenged with dengue virus 1; D2, PBMC challenged with dengue virus 2; D3, PBMC challenged with dengue virus 3. Panels show different scales according to the range of values observed.