What Is Sickle Cell Disease?

Reviewed by: HU Medical Review Board | Last reviewed: January 2021

Sickle cell disease (SCD) is a group of genetic red blood cell disorders. Normally, red blood cells are round and flexible. But people with SCD have rigid red blood cells that are C-shaped, like sickles or crescent moons. This causes red blood cells to die early and block blood vessels, leading to many complications.1

There has been great progress in diagnosing and treating SCD in recent years. This has reduced childhood mortality and increased life expectancy. But people with SCD still experience severe pain and discrimination because of the disease. They may suffer from depression and anxiety as a result.

What causes sickle cell disease?

SCD is a genetic disorder due to abnormal hemoglobin. Hemoglobin is a protein in red blood cells that carries oxygen throughout the body. People only show disease symptoms if they inherit 2 abnormal hemoglobin genes (1 from each parent). Other factors affect disease severity. SCD is a disease that is present from birth and does not develop later in life.2

A mutation in the hemoglobin gene causes hemoglobin proteins to clump together. This changes the shape of the red blood cell to look like a sickle. Sickle cells stick to blood vessel walls and block blood flow. This may prevent oxygen from reaching many parts of the body, causing attacks of severe pain.3

Because of their rigid shape, sickle cells break more often than normal healthy red blood cells and do not last as long in the bloodstream. Our bodies are always making new red blood cells but cannot keep up with the quick turnover of sickle cells. This means that people with SCD usually have fewer red blood cells and suffer from anemia (low red blood cell counts).3

What are the different types?

People with SCD have an abnormal type of hemoglobin called hemoglobin S (HbS) or “sickle hemoglobin.” They may also have another abnormal hemoglobin caused by different genetic mutations. People with SCD inherit a gene for HbS from 1 parent and a gene for any abnormal hemoglobin from the other parent.2

HbSS (Sickle cell anemia)

Sickle cell anemia is the most common and most severe type of sickle cell disease. It happens when a person inherits 2 genes for HbS (1 from each parent).


This is the second most common type of SCD. It occurs when someone inherits a gene for HbS and a gene for another abnormal hemoglobin called hemoglobin C. Symptoms of HbSC are milder than HbSS.

HbSD, HbSE, HbSO, and HbSβ-thalassemia

These are rare types of SCD. They occur when someone inherits a gene for HbS and a gene for another abnormal hemoglobin. This includes HbD, HbE, HbO, or Hbβ-thalassemia.

HbAS (Sickle cell trait)

This occurs when someone inherits a gene for HbS and a gene for normal hemoglobin A. It is called sickle cell trait because it usually does not cause symptoms. People with HbAS can pass on the gene for HbS to their children.

How common is this disease?

About 100,000 Americans have SCD. SCD occurs more often in people whose ancestors lived where malaria has been common, such as Africa and India. The disease is most common in people with West African heritage. This is because sickle cell trait protects against severe malaria infections.2,4

In the United States, most people who have SCD are Black. About 1 in 365 Black children are born with SCD, and 1 in 13 Black children are born with sickle cell trait. About 1 in 16,300 Hispanic children in the United States are born with SCD.3

What are the symptoms and complications?

People with SCD experience sudden, severe symptoms that last for a short period of time (acute), as well as symptoms that last for a long time, happen often, or get worse over time (chronic). Both acute and chronic SCD symptoms are due to blocked blood flow and low red blood cell count. People start showing signs around 5 months old. The earliest signs and symptoms are:3

SCD can damage many organs. Complications worsen over time because of repeated acute symptoms. Some common complications are:3

Some complications are life-threatening. You should see a doctor immediately if you have complications including:5

  • Fever
  • Episodes of severe pain
  • Swollen hands or feet
  • Abdominal swelling
  • Pale or yellow skin
  • Stroke symptoms

What are the outcomes for people with SCD?

Life expectancy and symptom management have greatly improved since the 1970s. Early diagnosis and prevention of childhood complications have been very important. Every U.S. state tests all newborns for SCD. Older children or adults can also be tested to find out if they have sickle cell trait.2,3

The only true cure for SCD is a blood and bone marrow transplant. A bone marrow transplant (or stem cell transplant) is only used in people with the most severe SCD and is not a common procedure. It is only considered if all other treatments fail to improve symptoms. Few people get this procedure because it requires a family member who is a matched donor. There are now several FDA-approved treatments that reduce the number of acute pain crises. Also, blood transfusions can be used to treat anemia, stroke, and chest crises.2,3

Despite these improvements, people with SCD still experience severe pain and symptoms that are difficult to control. This causes many people with SCD to suffer from anxiety and depression.1,3

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