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Sickle cell disease (SCD) is a disease of hemoglobin, a molecule that carries oxygen within red blood cells. In SCD, defective hemoglobin causes the red blood cells to become stiff instead of flexible and form a sickle or a crescent.

SCD is an inherited disease, meaning that the disease is passed on from parents to children through genes. Genes are instructions that tell the body how to make all the different substances the body needs to work properly. Since SCD is inherited, the disease is present at birth.

In SCD, a child inherits two defective hemoglobin genes (called hemoglobin S), one from each parent. This causes the body to make faulty hemoglobin. If a child inherits just one defective hemoglobin S gene, and the other hemoglobin gene is normal (hemoglobin A), he or she will have sickle cell trait, and will not have any SCD symptoms.

Crescent- or sickle-shaped red blood cells can get stuck in small blood vessels, blocking them. This keeps red blood cells and the oxygen they carry from getting to all parts of the body. Blocked blood flow can cause severe pain and organ damage. Complications of SCD can range from mild tiredness (fatigue) to more severe symptoms such as strokes, heart attacks, infections, and repeated episodes of severe pain.

In the United States, SCD affects about 70,000-100,000 people. It is most common among African Americans and Hispanics, according to the National Heart Blood and Lung Institute, but is also found in other ethnic and racial groups. Sickle cell anemia, where the patient inherited two hemoglobin S genes, is the most common form of sickle cell disease. There are variations of SCD where the patient inherits one hemoglobin S gene with another hemoglobin problem that results in a disease similar to classic SCD.

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