Stacked red blood cells, coloured scanning electron micrograph (SEM). These stacks of red blood cells, known as 'rouleaux', are abnormal and symptomatic of an underlying disease or condition. The stacking occurs because red blood cells have large flat surfaces that enable them to stick together. Conditions that lead to this include infections, inflammatory disorders, cancer, diabetes and anaemia. Magnification: x1800 when printed at 10 centimetres wide.