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In this paper, we investigate an optical-trap-based method for the detection of structural changes of the red blood cell (RBC) membrane affected by Ca2+ ions. Individual cells are immobilized by the use of optical tweezers and are monitored live, while the concentration of Ca2+ ions in the buffer is changed simultaneously. Ca2+ ions are known to affect the cells' membrane morphology. These changes are attributed to the formation of calcium-induced hydrophobic aggregates of phospholipid molecules in the RBC membrane, resulting in a net change in membrane rigidity. Membrane deformation results in the change of effective radius and the drag coefficient of the cell, both of which affect the Brownian motion of the cell in solution. This motion is indirectly measurable by monitoring the forward scattering light and its dependence on the size and drag coefficient of the cell. We show the relationship between the Ca2+ ion concentration and the optical trap specifications. The results are in agreement with previous biological studies and the phase contrast observations of living RBCs under investigation.