Buffy coat samples containing lymphocytes, monocytes, and granulocytes were obtained from the peripheral blood of 16 donors who had clinical manifestations of atopic hypersensitivity in their medical background. After ex vivo incubation with donor-specific allergens, the percentage of Band T-lymphocytes and natural killers remained unchanged. Incubation of buffy coat cells with allergens induced the production of IgE and IL-4 in all assayed samples. In 13 out of 16 cases, the reaction to contact with an allergen was also evident in the increasing of T-activated lymphocytes (CD3+, HLA-DR⁺) subpopulation. Cocultivation with mesenchymal stromal cells (MSCs) from bone marrow, adipose tissue and umbilical cord resulted in blocking of allergen-induced IgE and IL-4 secretion and increase in the subpopulation of HLA-DR+ T-lymphocytes. There was no significant difference in the effect of MSCs isolated from three different sources on allergenspecific responses of leukocytes. Coculturing of leukocytes with MSCs from all three sources increased the content of regulatory T-lymphocytes by approximately 30%. Thus, the immunomodulatory activity of MSCs in vitro results in blocking of the effector phase of allergic reactions.