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High power image of rouleaux formation in a patient with plasma cell leukemia. A circulating malignant plasma cell is present. Rouleaux formation refers to the stacking of 4 or more red blood cells. Red cell membranes have a negative charge (zeta potential) that causes red cells to repel each other. In the presence of increased positively charged plasma proteins such as fibrinogen or immunoglobulins, the negative charge on the red cell surface is diminished, allowing red cells to stick together.