Sickle Cell Disease - Risk Factors

Causes

Sickle Cell Disease and Hemoglobin

Sickle cell disease occurs from genetic changes that cause abnormalities in hemoglobin molecules:

• Hemoglobin A (HbA). HbA is the hemoglobin molecule found in normal red blood cells during childhood and adulthood. People who do not have sickle cell disease generally have this type of hemoglobin in their blood cells.
• Hemoglobin S (HbS). HbS (S is for sickle) is the abnormal variant of hemoglobin A, which occurs in sickle-red blood cells and is the primary characteristic of the disease. The difference between hemoglobin A (HbA) and hemoglobin S (HbS) is that only one protein out of about 300 are common to both. This protein lies along an amino-acid chain called beta-globin, where even a tiny abnormality has disastrous results.

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The Sickle Cell Disease Process

The symptoms and problems of sickle cell disease are a result of the hemoglobin S (HbS) molecule:

• When the sickle hemoglobin molecule loses its oxygen, it forms rigid rods called polymers that change the red blood cells into a sickle or crescent shape.
• These sickle-shaped cells stick to the walls and cannot squeeze through the capillaries. Blood flow through tiny blood vessels becomes slowed or stopped in many parts of the body. This deprives tissues and organs of oxygen.
• When this blood flow slows or stops suddenly in a certain part of the body, the decrease in oxygen (hypoxia) can cause severe pain (the sickle cell crisis). Over time, it leads to gradual destruction in organs and tissues throughout the body.

Click the icon to see an image of sickle cells.

• The higher the concentration of sickle hemoglobin and the more acidic the environment, the faster the sickle cell process is.
• When blood cells dry out (dehydrate), the density of hemoglobin S within the cell increases, thereby speeding the sickling process.
• Sickle cells also have a shorter life span (10 - 20 days) than normal red blood cells (90 - 120 days). Every day the body produces new red blood cells to replace old ones, but sickle cells become destroyed so fast that the body cannot keep up. The red blood cell count drops, which results in anemia. This gives sickle cell disease its more common name, sickle cell anemia.

Review Date: 01/13/2011
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

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