The trace element zinc is essential for immune function and its regulation. Since zinc deficiency and allergic hyperresponsive reactions are often accompanied, the influence of zinc on allergen-induced cell growth, CD4+ regulatory T (Treg) cell numbers and cytokine expression during allergic immune reactions was investigated. Peripheral blood mononuclear cells (PBMCs) from non-atopic and atopic subjects were treated with timothy grass allergen pre-incubated with or without zinc. Proliferation was determined by analyzing the incorporation of H-3-thymidine. Intracellular zinc and Foxp3 levels and cell surface antigens were measured by FACS, cytokine expression by ELISA and real-time PCR. Incubation with 50 mu M zinc sulfate (Zn50) enhances cytosolic zinc concentrations in CD3+ T cells. The data also reveal that the combination of Zn50 plus allergen significantly reduces PBMC proliferation of atopic subjects. Additionally, Zn50 plus allergen enhances Th1 cytokine responses shown by increased interferon (IFN)-gamma/interleukin (IL)-10 ratios as well as enhanced tumor necrosis factor-alpha release. In response to allergen, zinc increases Treg cells and upregulates the mRNA expression of cytotoxic T-lymphocyte antigen-4 in atopic subjects. Interestingly, Zn50 alone leads to an increase of CD4+CD25high(hi)+ cells in atopic and non-atopic subjects. Zinc may regulate unwanted hyperresponsive immune reactions by suppressing proliferation through a significant shift from IL-10 to the Th1 cytokine IFN-gamma, and enhanced regulatory T cell numbers. Therefore, zinc supplementation may be a promising tool for the therapy of allergies, without negatively affecting the immune system.