Sickle cells, also referred to as drepanocytes, are formed as a result of the presence of hemoglobin S in the red cell. As the red cell ages, it becomes rigid as it passes through the low oxygen tension atmosphere of the small capillaries in the body. In the absence of oxygen, hemoglobin S polymerizes into rods, causing the sickle cell shape. The shape of sickle cells can vary from cigar-shaped, as shown in the upper image, to the more severe sickled-form, shown in the bottom image.