Understanding sickle cell disease
Everything from the story, explained clearly in one place, plus a simulation you can control.
What is sickle cell disease?
[[sickle-cell-disease|Sickle cell disease]] is an inherited blood disorder. It is caused by a change in the [[hbb-gene|HBB gene]], which holds the instructions for part of [[haemoglobin]], the protein that carries oxygen in [[red-blood-cell|red blood cells]].
The changed gene makes [[haemoglobin-s|haemoglobin S]]. After releasing oxygen, haemoglobin S can clump into stiff fibres ([[polymerisation]]). This bends red blood cells into a curved sickle shape. Sickled cells are rigid and sticky, can block small blood vessels, and die after 10 to 20 days instead of about 120.
A person has sickle cell disease when they inherit an S gene from both parents (genotype SS). With one S gene (AS), a person has [[sickle-cell-trait|sickle cell trait]] and usually has no symptoms. Other forms exist too, such as SC disease, where S is inherited with another haemoglobin change.
Blood flow simulation
Move the slider to change how many red blood cells are sickled. Normal cells bend through the narrow capillary. Rigid sickled cells can get trapped, and every trapped cell slows the flow for all the others.
How this model works
This is a simplified educational model, not a medical prediction. The share of sickled cells sets how many can become trapped in the narrow section. Blood flow is calculated as 1 − 0.85 × (trapped cells ÷ maximum trapped cells), so a fully jammed capillary still lets about 15% through.
Real blood flow also depends on vessel size, blood thickness (viscosity), how sticky cells are to vessel walls, inflammation and hydration. Engineers and scientists build models like this, with many more variables, to study diseases and test treatments.
Signs and symptoms
Symptoms usually begin in early childhood, often around 5 to 6 months of age, and vary a lot from person to person.
- Pain crises: sudden pain in bones, joints, chest or belly when sickled cells block blood flow ([[vaso-occlusion]]).
- [[anaemia|Anaemia]]: tiredness, weakness, pale skin and shortness of breath.
- Swelling of the hands and feet, often one of the first signs in babies.
- Frequent infections, because the spleen is damaged.
- Yellow eyes or skin (jaundice), from red blood cells breaking down quickly.
- Delayed growth in some children and teenagers.
Common triggers
These can make sickling more likely. Avoiding them does not cure the disease, but it can reduce crises.
Dehydration
Not drinking enough water, especially in hot weather or during sport.
Low oxygen
Hard exercise without rest, high altitude, or breathing problems.
Temperature extremes
Getting very hot, very cold, or moving suddenly between the two, like jumping into cold water.
Infection and stress
Illnesses such as malaria or flu, and physical or emotional stress.
Emergency warning signs
For someone with sickle cell disease, these need medical care right away:
- Fever (38.5°C / 101°F or higher)
- Chest pain, coughing or trouble breathing
- Severe headache, sudden weakness, trouble speaking or a drooping face (possible stroke)
- Severe pain that does not get better with usual medicine
- A suddenly swollen belly, or looking very pale and tired
- A painful erection lasting more than a few hours
Treatment and care
There is a lot doctors can do, and care starts best when the disease is found early through [[newborn-screening|newborn screening]].
- [[hydroxyurea|Hydroxyurea]]: a daily medicine that raises [[fetal-haemoglobin|fetal haemoglobin]] and reduces crises.
- Folic acid: helps the body make new red blood cells.
- Penicillin and vaccines: protect young children from serious infections.
- Malaria prevention: sleeping under treated bed nets is especially important.
- [[blood-transfusion|Blood transfusions]]: for severe anaemia, stroke prevention and some complications.
- [[stem-cell-transplant|Stem cell transplant]]: can cure the disease, but needs a matched donor and has serious risks.
- [[gene-therapy|Gene therapy]]: approved in the United States in 2023; very expensive and not yet widely available.
Myths and facts
Myth
You can catch sickle cell disease from someone who has it.
Fact
It is inherited through genes. It is not contagious at all.
Myth
It is caused by a curse, a spirit or something the mother did.
Fact
It happens when a child inherits an S gene from both parents. Nobody is to blame.
Myth
People with sickle cell trait (AS) have the disease.
Fact
People with AS are carriers and usually healthy, but they can pass S to their children.
Myth
People with sickle cell can’t go to school, work or play sport.
Fact
With good care, hydration and rest when needed, they can live full, active lives.
Myth
Children with sickle cell always die young.
Fact
Early diagnosis, infection prevention and modern treatment help many people live into adulthood and old age.
Know your genotype
A simple blood test can show whether you are AA, AS or SS. Knowing your genotype, and your partner’s, helps couples understand the chances for their future children and make informed decisions together.
Two people with AS have a 1 in 4 chance of having a child with sickle cell disease in each pregnancy. Ask a doctor, clinic or health worker about genotype testing.
See the chances in the Inheritance Lab →