Drepanocytes or sickle cells are formed as a result of the presence of hemoglobin S in the red cell. As the red cell ages, it becomes less flexible or deformable and 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. Sickle cells can be somewhat pointed at the ends, as the cell shown in the center, or more rounded or cigar-shaped, as the one slightly to the left. Several sickle cells are indicated by S, and a target cell or dacrocyte is indicated by T in the smear shown on the right.