Sickle cell anemia is an inherited disease that begins at birth and continues for life. Sickle cell anemia causes the body to manufacture red blood cells with abnormal hemoglobin. Hemoglobin is the oxygen-carrying part of a red blood cell, or erythrocyte. The main type of hemoglobin in normal red cells is called hemoglobin A. People with sickle cell anemia have a defective type of hemoglobin called hemoglobin S (for sickle).
Normal red cells are round and flexible, gliding easily through the blood vessels. They don't stick to each other or to the sides of the vessels. In sickle cell anemia, under certain circumstances, the molecules of hemoglobin S stick together and form little crystals. The crystals gravitate to one side of the cell, which collapses the other side. The cells become stiff and distorted in appearance. Some cells resemble a crescent or farmer's sickle, while others may resemble teardrops or even stars.
These misshapen cells stick to each other, and because of a chemical on their surface, they also stick the vessel walls. As the flow of blood enters the body's tiniest vessels, the capillaries, these sticky, rigid, odd-shaped cells create a logjam. Blood flow is blocked and body tissues are deprived of their necessary oxygen. A person with sickle cell anemia experiences this as a painful crisis.
Normal red blood cells live about 120 days. Sickle-shaped red cells don't live nearly as long--only 15 - 25 days. New red cells are created daily, but in sickle cell anemia the body can't replace the dying red cells fast enough. There is always a shortage of red cells, and therefore a deficit in hemoglobin. Low hemoglobin levels define the condition known as anemia, or when S hemoglobin is involved, sickle cell anemia.
The terms "sickle cell anemia" and "sickle cell disease" are both used to refer to the same condition. There are actually three variations of sickle cell disease, with sickle cell anemia being the most common. In the other types, a person inherits hemoglobin S plus another type of defective hemoglobin. One of the other types of sickle cell disease is called sickle cell Thalassemia disease, or S Thal. The other kind is called sickle cell hemoglobin C disease, or SC disease. SC disease is the mildest form of sickle cell disease.