What is sickle cell disease (SCD)?

SCD is a serious group of conditions which are inherited (genetic). It affects the red blood cells in the blood.
Sickle cell anaemia is the name of a specific form of SCD in which there are two sickle cell genes (see below).

With SCD, the red blood cells have a tendency to go out of shape and become sickle-shaped (like a crescent
moon) – instead of their normal disc shape. This can cause various problems – as described later. In between the
episodes of illness, people with SCD feel well.

SCD is therefore a group of conditions that cause red cells to become sickle-shaped.

Sickle cell trait is not the same as SCD or sickle cell anaemia.

Sickle cell trait means you carry a sickle cell gene, but it does not normally cause illness. See separate leaflet called Sickle Cell Trait
(Sickle Cell Carrier) and Sickle Cell Screening Tests for more information.

The rest of this leaflet will discuss SCD, which includes sickle cell anaemia and the other less common disorders.

What causes sickle cell disease (SCD)?

The cause is inherited (genetic). It is a change in the genes which tell the body how to make an important protein
called haemoglobin. To get SCD, you need to have two altered haemoglobin genes, one from each parent. If you
only have one of these genes, you will have sickle cell trait, which is very much milder.

What are the symptoms of sickle cell disease (SCD)?

Symptoms of SCD come and go. Usually there are bouts (episodes) of symptoms but, in between episodes, you
feel well. The reason that symptoms come and go is that the red blood cells can behave normally for much of the
time – but if something makes too many of them sickle, the sickle cells cause symptoms. If there are severe and
sudden symptoms due to sickling, this is called a sickle cell crisis.


There is a lot of individual variation in symptoms – how many and how often you get them. Some people with
SCD have frequent symptoms, while others have very few and their SCD is hardly noticeable. For most people,
symptoms are somewhere in between these two extremes. Most people with SCD have a few episodes of sickle
cell crisis each year.


Symptoms usually begin after a few months of age. (Before that age, the baby has a different haemoglobin,
called fetal haemoglobin, which is not affected by the sickle cell gene.)

Learn more about sickle cell disease

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