Protein levels of plated spleen cells. Protein levels (pg/ml) of TNF-α and IL-13 in the supernatant of plated spleen cells extracted from allergic mice exposed to nTiO2 for 2 hours a day, three days a week, for four weeks at a concentration of 10 mg/m3 or left unexposed (-). Cells were either stimulated with OVA or left untreated. N = 8 mice per group. The bars represent mean ± SEM; * P < 0.05 significantly different from unexposed control; Mann-Whitney U test.