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What Is Sickle Cell Disease?

The term sickle cell disease (SCD) describes a group of inherited red blood cell disorders. People with SCD have abnormal hemoglobin, called hemoglobin S or sickle hemoglobin, in their red blood cells.

Hemoglobin is a protein in red blood cells that carries oxygen throughout the body.

“Inherited” means that the disease is passed by genes from parents to their children. SCD is not contagious. A person cannot catch it, like a cold or infection, from someone else.

People who have SCD inherit two abnormal hemoglobin genes, one from each parent. In all forms of SCD, at least one of the two abnormal genes causes a person’s body to make hemoglobin S. When a person has two hemoglobin S genes, Hemoglobin SS, the disease is called sickle cell anemia. This is the most common and often most severe kind of SCD.

Hemoglobin SC disease and hemoglobin Sβ thalassemia (thal-uh-SEE-me-uh) are two other common forms of SCD.

Some Forms of Sickle Cell Disease

  • Hemoglobin SS
  • Hemoglobin SC
  • Hemoglobin Sβ0 thalassemia
  • Hemoglobin Sβ+ thalassemia
  • Hemoglobin SD
  • Hemoglobin SE


Cells in tissues need a steady supply of oxygen to work well. Normally, hemoglobin in red blood cells takes up oxygen in the lungs and carries it to all the tissues of the body.

Red blood cells that contain normal hemoglobin are disc shaped (like a doughnut without a hole). This shape allows the cells to be flexible so that they can move through large and small blood vessels to deliver oxygen.

Sickle hemoglobin is not like normal hemoglobin. It can form stiff rods within the red cell, changing it into a crescent, or sickle shape.

Sickle-shaped cells are not flexible and can stick to vessel walls, causing a blockage that slows or stops the flow of blood. When this happens, oxygen can’t reach nearby tissues.

Normal Red Cells and Sickle Red Cells

An image that shows the difference between a normal red blood cell verses  a sickle cell with abnormal (sickle) hemoglobin
Figure A shows normal red blood cells flowing freely in a blood vessel. The inset image shows a cross-section of a normal red blood cell with normal hemoglobin. Figure B shows abnormal, sickled red blood cells blocking blood flow in a blood vessel. The inset image shows a cross-section of a sickle cell with abnormal (sickle) hemoglobin forming abnormal stiff rods.

The lack of tissue oxygen can cause attacks of sudden, severe pain, called pain crises. These pain attacks can occur without warning, and a person often needs to go to the hospital for effective treatment.

Most children with SCD are pain free between painful crises, but adolescents and adults may also suffer with chronic ongoing pain.

The red cell sickling and poor oxygen delivery can also cause organ damage. Over a lifetime, SCD can harm a person’s spleen, brain, eyes, lungs, liver, heart, kidneys, penis, joints, bones, or skin.

Sickle cells can’t change shape easily, so they tend to burst apart or hemolyze. Normal red blood cells live about 90 to 120 days, but sickle cells last only 10 to 20 days.

The body is always making new red blood cells to replace the old cells; however, in SCD the body may have trouble keeping up with how fast the cells are being destroyed. Because of this, the number of red blood cells is usually lower than normal. This condition, called anemia, can make a person have less energy.


Sickle cell disease is a life-long illness. The severity of the disease varies widely from person to person.

In high-income countries like the United States, the life expectancy of a person with SCD is now about 40–60 years. In 1973, the average lifespan of a person with SCD in the United States was only 14 years. Advances in the diagnosis and care of SCD have made this improvement possible.

At the present time, hematopoietic stem cell transplantation (HSCT) is the only cure for SCD. Unfortunately, most people with SCD are either too old for a transplant or don’t have a relative who is a good enough genetic match for them to act as a donor. A well-matched donor is needed to have the best chance for a successful transplant.

There are effective treatments that can reduce symptoms and prolong life. Early diagnosis and regular medical care to prevent complications also contribute to improved well-being.


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Last Updated: June 12, 2015