It is a blood disorder that causes anemia (shortage of red blood cells) and periodic pain. In people with sickle cell anemia, not all of their hemoglobin (a molecule in red blood cells that carries oxygen to other parts of the body) works properly. Some of the hemoglobin forms long, rod-like structures that cause the red blood cells to be sickle-shaped and stiff. These cells can clog small blood vessels, preventing some organs or tissues from receiving enough oxygen. When this happens, it can cause episodes of severe pain or damage organs and tissues, and may lead to other serious medical problems.