Defective hemoglobin is responsible for sickle cell anemia. Hemoglobin is a red, iron-rich protein that gives blood its red color. It's a component of every red blood cell in your body. Hemoglobin enables red blood cells to carry oxygen from your lungs to all parts of your body, and to carry carbon dioxide waste from other parts of your body to your lungs so that it can be exhaled.

Red blood cells with normal hemoglobin are smooth and round and glide through blood vessels. But, after delivering oxygen, red blood cells with sickle cell hemoglobin may become hard, sticky and shaped like a sickle used to cut wheat. These crescent-shaped cells — sickle cells — can get stuck in small blood vessels, blocking blood flow and causing episodes of pain and damage to organs.

Your bone marrow — the red, spongy material found within the cavity of many of your large bones — regularly produces red blood cells. Bone marrow also produces white blood cells to fight infections and platelets to help blood clot. These two types of blood cells aren't involved in sickle cell anemia.

Once red blood cells leave the bone marrow, they normally live for about three to four months before they die and need to be replaced. To produce new red blood cells, your bone marrow must manufacture lots of hemoglobin.

An inherited disease
Sickle cell anemia is an inherited condition. The cause of sickle cell anemia is an error in the gene that tells your body to make hemoglobin. This error in the gene results in a defective form of hemoglobin.

Normal red blood cells contain hemoglobin A. People with sickle cell anemia make hemoglobin S — the S stands for sickle.

If you have sickle cell anemia, you have inherited a sickle cell gene — the defective gene responsible for sickle cell anemia — from each of your parents. It takes two genes, one from each parent, to cause the disease.

Some people inherit a sickle cell gene from just one parent. These people are said to carry the sickle cell trait. Their bodies make both normal hemoglobin and sickle cell hemoglobin. Their blood may contain some sickle cells, but they usually don't experience symptoms unless they're in an area with low oxygen — such as at high altitudes on an airplane or on a mountain. The main risk is that they may pass the gene on to their children.

Researchers believe the defective hemoglobin gene that causes sickle cell anemia developed from a genetic mutation many years ago among people living in parts of Africa, the Mediterranean, the Middle East and India. At that time, malaria epidemics killed many people in those regions.

But, some people in those regions had a genetic mutation that caused some of their red blood cells to change shape. Sickle cells interfere with the growth of the parasite that causes malaria, and those with the mutation often survived malaria outbreaks.

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Over time, the survivors migrated, had children and, in some cases, carriers of the gene married one another. Some of their children inherited two copies of the mutated gene. Although inheriting one copy of the mutated gene confers some benefits, inheriting two copies results in sickle cell anemia.

Today, millions of people all over the world have sickle cell anemia. It's particularly common among people with African, Spanish, Mediterranean, Middle Eastern and Indian ancestry.

In West and Central Africa, as many as one in four people have the sickle cell trait. In the United States, about 1,000 infants are diagnosed with sickle cell anemia every year. By comparison, in Nigeria — a country whose population is less than half that of the United States — between 45,000 and 90,000 babies are born with sickle cell anemia annually.

For a baby to be born with sickle cell anemia, both parents have to carry the sickle cell gene.

If two people who each carry one sickle cell gene have a child, the chances are:

• One in four that the child will have normal hemoglobin

• Two in four that the child will carry one sickle cell gene

• One in four that the child will be born with sickle cell anemia

Some people know from family history or from having the disease that they carry the sickle cell gene. In more than 40 states, testing for the gene is part of the usual round of tests done on newborns at the hospital.

Doctors diagnose sickle cell anemia through a blood test that checks for hemoglobin S — the defective form of hemoglobin that underlies the disease.

Your doctor draws a sample of your blood and sends the sample to a laboratory. At the laboratory, a technician performs a test called hemoglobin electrophoresis. This test identifies the type of hemoglobin you have. Hemoglobin A is normal hemoglobin. Hemoglobin S is sickle cell hemoglobin. Other types of abnormal hemoglobin, which lead to other forms of sickle cell disease, include hemoglobin C, D and E.

If your blood shows only hemoglobin A, you're healthy. If you have both hemoglobin A and some hemoglobin S, you carry the sickle cell gene. If you have only hemoglobin S, the diagnosis is sickle cell anemia.

To confirm the diagnosis, a sample of your blood is also examined under a microscope to check for large numbers of sickle cells — a marker of the disease. You'll also undergo a blood test to check for anemia — a low red blood cell count.

You may have additional tests at your doctor's office or at a hospital to rule out other conditions. Sickle cell anemia is diagnosed in most people when they're a newborn or infant. However, some people are older and develop symptoms before receiving the diagnosis of sickle cell anemia.

It's possible to detect sickle cell anemia in an unborn baby. By sampling some of the fluid around the baby in the mother's womb (amniotic fluid) or some of the placenta, doctors can tell whether a fetus has sickle cell anemia or carries the sickle cell gene.

If you have sickle cell anemia, you likely will be referred to a doctor who specializes in blood-related diseases (hematologist). If you or your child carries the sickle cell gene, you may be referred to a genetic counselor — an expert in genetic diseases.

Sickle cell anemia can lead to a host of complications, including:

• Stroke. A stroke can occur if sickle cells block blood flow to an area of your brain. Stroke is one of the most serious complications of the disease. Signs of stroke include seizures, weakness or numbness of your arms and legs, sudden speech difficulties, and loss of consciousness. If your baby or child has any of these signs and symptoms, seek medical treatment immediately. A stroke can be fatal.

• Acute chest syndrome. This life-threatening complication of sickle cell anemia causes chest pain, fever and difficulty breathing. Acute chest syndrome is similar to pneumonia, but is caused by a lung infection or trapped sickle cells in your lungs. It requires emergency medical treatment with antibiotics, blood transfusions and drugs that open up airways in your lungs. Recurrent attacks can damage your lungs.

• Organ damage. Sickle cells can block blood flow through blood vessels, immediately depriving an organ of blood and oxygen. In sickle cell anemia, blood is also chronically low on oxygen. Chronic deprivation of oxygen-rich blood can damage nerves and organs in your body, including your kidneys, liver and spleen. Organ damage can be fatal.

• Blindness. Tiny blood vessels that feed your eyes can get plugged with sickle cells. Over time, this can damage the retina — the portion of each eye that processes visual images — and lead to blindness.

• Other complications. Sickle cell anemia can cause open sores, called ulcers, on your legs. Sickle cells can block blood vessels that nourish your skin, causing skin cells to die. Once skin is damaged, sores can develop. Gallstones also are a possible complication. The breakdown of red blood cells produces a substance called bilirubin. Bilirubin is responsible for yellowing of the skin and eyes (jaundice) in people with sickle cell anemia. A high level of bilirubin in your body can also lead to gallstones. Men with sickle cell anemia may experience painful erections, a condition called priapism. Sickle cells can prevent blood flow out of an erect penis. Over time, priapism can damage the penis and lead to impotence in men with sickle cell anemia.