Introduction:

Sickle cell disease (also called sickle cell anemia) is an inherited blood disorder that affects red blood cells. The sickle cell gene causes the body to produce abnormal hemoglobin. In sickle cell disease, the hemoglobin clumps together, which causes red blood cells to become stiff and develop a C-shaped (“sickle”) form. These sickled red blood cells can block blood vessels, reducing blood flow in many parts of the body. This process results in tissue and organ damage.

Hemoglobin and Iron

Each red blood cell contains about 280 million hemoglobin molecules. Hemoglobin is the most important component of red blood cells. It is composed of protein (globulin) and a molecule (heme), which binds to iron.

In the lungs, the heme component takes up oxygen and releases carbon dioxide. The red blood cells carry the oxygen to the body's tissues, where the hemoglobin releases the oxygen in exchange for carbon dioxide, and the cycle repeats. The oxygen is essential for all cells in the body to function.

Sickle cell disease reduces or denies adequate oxygen to many parts of the body. This contributes to the severe pain experienced as a sickle cell crisis and both short- and long-term organ damage.

Sickle Cell Disease and Hemoglobin

Sickle cell disease occurs from genetic changes which causes a portion of the hemoglobin molecules to be abnormal:

• Hemoglobin A (HbA). HbA is the hemoglobin molecule found in normal red blood cells during childhood and adulthood. People without sickle cell anemia have primarily 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) lies in only one protein out of about 300 that are common to both. This protein lies along an amino-acid chain called beta-globin, where even a tiny abnormality has disastrous results.

Hemoglobin is the most important component of red blood cells. It is composed of a protein called heme, which binds oxygen. In the lungs, oxygen is exchanged for carbon dioxide. Abnormalities of an individual's hemoglobin value can indicate defects in red blood cell balance. Both low and high values can indicate disease states.

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.

• The higher the concentration of sickle hemoglobin and the more acidic the environment, the faster the sickle cell process.
• 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 that of 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.