Expansion of the Th17 cell population correlates with the depletion of natural regulatory T cells. PBMCs from healthy controls (n = 10) and AA patients at diagnosis (n = 21), in CR (n = 12), or in PRs (n = 15) were stained with anti-CD3, anti-CD4, anti-CD25 antibodies followed by intracellular FOXP3 antibody and examined by flow cytometry. Correlation between CD4⁺CD25^highFOXP3⁺ T cells (natural regulatory T cells) and CD8⁻ IL-17⁺ T cells (Th17 cells) was studied according to the stage of the disease. Frequencies of natural regulatory T cells and Th17 cells are indicated in healthy controls (●) and in AA at diagnosis (▵), in complete remission (▴), and in PR AA (○).