Red blood cell rouleaux. Computer illustration of stacks (rouleaux) of red blood cells (erythrocytes) inside a blood vessel. Rouleaux are stacks or aggregations of red blood cells that form due to the unique disc shape of the cells. Rouleaux form when the plasma protein concentration is high and may be a non-specific indicator of disease. They may also be a cause of disease itself as they can restrict blood flow through the body.