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Calcium plays a fundamental role in many essential functions of a living organism. It is well known that many infections can reduce extracellular calcium concentrations ([Ca]) (hypocalcaemia) in human blood. The importance of Ca for the T lymphocyte activation has been known for a long time, but the optimal range of [Ca] levels has not been found yet. The main goal of this work was the study of functional activities of CD4 cells in human peripheral blood (HPB) in vitro in the conditions of hypocalcemia. Polyclonal activation was induced in whole blood of healthy donors by immobilized monoclonal antibodies (mAbs) toward CD3 molecules (epsilon chain of the T cell receptor) and dissolved mAbs toward CD28 (the major molecule of T cell costimulation). Dependencies of T-cell activation parameters on varied [Ca] in HPB in vitro were studied. Production of interleukins-2 (IL-2), IL-4, IL-17A, interferon-gamma (IFN-γ), and tumor necrosis factor-alpha (TNF-α) as well as CD4CD69, CD4CD25, and CD4(CYTOKINE) T cells were used as activation parameters of the T cell polyclonal activation in HPB. EGTA was used as a tool for the reduction of [Ca] levels. The results demonstrated that the EGTA-stimulated binding of extracellular calcium in HPB caused a nonlinear dependence of T cell activation on [Ca]. No EGTA influence on resting T cells was found. The dependence of the number of CD4CD69 T cells on [Ca] was represented as a nonlinear curve with two maxima and a local minimum in between. One of the maxima was in the area of the HPB lower [Ca] normal level, whereas the other one was even lower, which is characteristic for pathological states. The T cell-induced production of IL-2, IL-4, IL-17A, IFN-γ, and TNF-α as well as the populations of CD4CD25 and CD4(CYTOKINE) T cells were analyzed in HPB in vitro under the same activation conditions. All the dependencies obtained were similar in shape to that of the CD4CD69 T cell population versus EGTA concentration in HPB in vitro and was observed as curves with two maxima. The dependencies of this type were described first. It is likely that the presence of two maxima and a local minimum in between for the dependence of HPB T cell activation parameters on [Ca] concentrations is of great physiological importance because hypocalcemia has been associated with many pathological states. [ABSTRACT FROM AUTHOR]