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 B- and T-lymphocytes and natural killers (NK) remained unchanged. Buffy coat incubation with allergens induced production of IgE and IL-4 in all studied 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. Co-cultivation with MSC from bone marrow, adipose tissue and umbilical cord resulted in blocking of allergen-induced IgE and IL4 secretion and HLA-DR+ T-lymphocytes subpopulation increase. There were no significant differences in the effect of MSCs, isolated from three different sources, on allergen-specific responses of leukocytes. Co-culturing of leukocytes with MSCs from all three sources led to an increase in the content of regulatory T-lymphocytes by an average of 30%. Thus, the immunomodulatory activity of MSCs in vitro results in blocking of the effector part of allergic reactions.