A method for isolating a target cell having a surface marker on a cell surface from a biological sample, the method comprising: a-1) contacting particles comprising at least one polymer having negative charges and at least one antibody bound to the polymer with the biological sample in a solution wherein the antibody specifically binds to a surface marker of at least one target cell; b-1) adding particles comprising at least one polymer having positive charges into the mixed solution of step a-1); c-1) adjusting a pH value of the mixed solution of step b-1), and d-1) isolating the target cell from the mixed solution of step c-1); or a-2) contacting particles comprising at least one polymer having positive charges and at least one antibody bound to the polymer with the biological sample in a solution wherein the antibody specifically binds to a surface marker of at least one target cell; b-2) adding particles comprising at least one polymer having negative charges into the mixed solution of step a-2); c-2) adjusting a pH value of the mixed solution of step b-2), and d-2) isolating the target cell from the mixed solution of step c-2), wherein the pH value of the solution is adjusted in step c-1) or step c-2) to a value higher than a pKa value of the polymer having negative charges and lower than a pKa value of the polymer having positive charges, such that an electrostatic attraction forms between the particles having negative charges and the particles having positive charges, and thus aggregation occurs between the particles, thereby increasing the overall size of the target cell, wherein the increase in overall size allows differentiation of the target cell from the other cells and isolation of the target cell in step d-1) or d-2). a-1) contacting particles comprising at least one polymer having negative charges and at least one antibody bound to the polymer with the biological sample in a solution wherein the antibody specifically binds to a surface marker of at least one target cell; b-1) adding particles comprising at least one polymer having positive charges into the mixed solution of step a-1); c-1) adjusting a pH value of the mixed solution of step b-1), and d-1) isolating the target cell from the mixed solution of step c-1); or a-2) contacting particles comprising at least one polymer having positive charges and at least one antibody bound to the polymer with the biological sample in a solution wherein the antibody specifically binds to a surface marker of at least one target cell; b-2) adding particles comprising at least one polymer having negative charges into the mixed solution of step a-2); c-2) adjusting a pH value of the mixed solution of step b-2), and d-2) isolating the target cell from the mixed solution of step c-2), wherein the pH value of the solution is adjusted in step c-1) or step c-2) to a value higher than a pKa value of the polymer having negative charges and lower than a pKa value of the polymer having positive charges, such that an electrostatic attraction forms between the particles having negative charges and the particles having positive charges, and thus aggregation occurs between the particles, thereby increasing the overall size of the target cell, wherein the increase in overall size allows differentiation of the target cell from the other cells and isolation of the target cell in step d-1) or d-2).