Under the microscope, red blood cells from a patient with sickle cell anemia are sickle-shaped, meaning that they look like crescents, according to the National Heart, Lung and Blood Institute. Normally, red blood cells are shaped like doughnuts without holes, or biconcave disks.
The difference in red blood cell shape is due to an abnormal type of hemoglobin, a protein that carries oxygen, explains the National Heart, Lung and Blood Institute. In sickle cell patients, the abnormal hemoglobin clumps together to form rods, leading to the sickle-shaped red blood cells. Unlike normal red blood cells, these sickle-shaped cells stick to the walls of blood vessels and lead to blockages. The blocked blood flow can lead to painful episodes and organ damage.