Erythrocyte aggregation is the reversible clumping of these cells under low shear forces or at stasis. Erythrocytes aggregate in a special way, forming rouleaux. Rouleaux are stacks of erythrocytes which form because of the unique discoid shape of the cells in vertebrate body. The flat surface of the discoid RBCs give them a large surface area to make contact and stick to each other; thus, forming a rouleau. Rouleaux formation takes place only in suspensions of RBC containing high-molecular, fibrilar proteins or polymers in the suspending medium (often Dextran-2000 in-vitro). The most important protein causing rouleaux formation in plasma is fibrinogen. RBC suspended in simple salt solutions do not form rouleaux.