Sickle cell anemia is an inherited blood disorder characterized by defective hemoglobin (a protein in red blood cells that carries oxygen to the tissues of the body).
Sickle cell anemia inhibits the ability of hemoglobin in red blood cells to carry oxygen. Cells containing normal hemoglobin are smooth, disk-shaped, and flexible, like doughnuts without holes, so they can move easily through the vessels in our bodies. Cells containing sickle cell hemoglobin are stiff and sticky and form into the shape of a sickle or crescent, like the letter C, when they lose their oxygen. These sickle cells tend to cluster together, and cannot easily move through the blood vessels. The cluster causes a blockage in small arteries or capillaries and stops the movement of healthy, normal oxygen-carrying blood. This blockage is what causes the painful and damaging complications of sickle cell disease.
Sickle cells only live for about 10 to 20 days, while normal red blood cells can live up to 120 days. Also, sickle cells risk being destroyed by the spleen because of their shape and stiffness. The spleen is an organ that helps filter infection out of the blood. Sickled cells get stuck in this filter and die. Due to the decreased number of healthy red blood cells circulating in the body, a person with sickle cell disease is chronically anemic. The spleen also suffers damage from the sickled cells blocking healthy oxygen carrying cells and typically shrinks and becomes inactive in the first few years of life. Without a normal functioning spleen, these individuals are more at risk for infections. Infants and young children are at risk for life-threatening infections.
The most common variations of sickle cell anemia include the following:
- Sickle cell trait. The person is carrying a single defective gene. Some of their hemoglobin is the destructive HbS, but they also have mostly normal adult hemoglobin, HbA. People with sickle cell trait are usually without symptoms of the disease. Mild anemia may occur and red blood cells tend to be small. Under intense, stressful conditions, exhaustion, hypoxia (low oxygen), and/or severe infection, the sickling of the defective hemoglobin may occur and result in some complications associated with the sickle cell anemia. Most people with the sickle cell trait lead normal lives.
- Sickle cell anemia. The person has most or all of the normal hemoglobin (HbA) replaced with the sickle hemoglobin (HbS). This is referred to as HbSS. It is the most common and most severe form of the sickle cell variations. These people suffer from a variety of complications due to the shape and thickness of the sickled cells. Severe and chronic anemia is also a common characteristic for people with HbSS.
- Sickle cell--hemoglobin SC disease. The person has one copy of both HbS and HbC. This is often referred to as HbSC. Hemoglobin C causes abnormal red blood cells, called target cells, to develop. Having just some hemoglobin C and normal hemoglobin, a person will usually not have any symptoms of anemia, but can develop anemia and eye and hip complications later in life. However, if a person has both HbS and HbC, some mild to moderate anemia occurs. These people often suffer some of the complications associated with HbSS, sickle cell disease, but to a milder degree. Vaso-occlusive crises (the flow of blood is blocked because the sickled cells have become stuck in the blood vessels), organ damage from repeated sickling and anemia, and high risk for infection are all similar traits for HbSS and HbSC.
- Sickle cell--hemoglobin E disease. This variation is similar to sickle cell-C disease except that an element has been replaced in the hemoglobin molecule. This variation is most often also seen in Southeast Asian populations. Some people with hemoglobin E disease are without symptoms. However, under certain conditions, such as exhaustion, hypoxia, severe infection, and/or iron deficiency, some mild to moderate anemia may occur.
- Hemoglobin S-beta-thalassemia. This involves an inheritance of both the thalassemia and sickle cell genes. The disorder produces symptoms of moderate to severe anemia and many of the same conditions associated with sickle cell anemia. While this disorder more often has milder symptoms than sickle cell anemia, it may also produce complications as severe as those of that disease.
All forms of sickle cell anemia can exhibit the complications associated with the disease.
Who is affected by sickle cell anemia?
Sickle cell anemia primarily affects those of African descent and Hispanics of Caribbean ancestry, but the trait has also been found in those with Middle Eastern, Indian, Latin American, American Indian, and Mediterranean heritage.
It has been estimated that 90,000 to 100,000 people in the U.S. are affected by sickle cell anemia and that approximately 3 million people have the sickle cell trait. Approximately one in 12 African-Americans has sickle cell trait. Millions worldwide suffer complications from sickle cell disease.