Sickle cell anemia (SCA) is a genetic blood disorder affecting hemoglobin, the protein in red blood cells that carries oxygen. Individuals with SCA produce an abnormal type of hemoglobin, causing red blood cells to become stiff, sticky, and crescent-shaped, resembling a sickle. These misshapen cells interfere with normal blood flow, leading to various health complications. SCA is a fixed genetic condition present from conception and cannot be acquired later in life.
The Inherited Nature of Sickle Cell Disease
Sickle cell anemia is strictly inherited, passed down through genes from parents to a child. The disorder is caused by a specific mutation in the HBB gene, which provides instructions for making the beta-globin chain of hemoglobin. This mutation leads to the production of abnormal hemoglobin S (HbS) instead of the typical adult hemoglobin A (HbA).
SCA follows an autosomal recessive inheritance pattern, requiring a child to receive two copies of the altered HBB gene—one from each parent—to develop the full disease. If both parents carry one copy of the gene, there is a 25% chance with each pregnancy that their child will inherit two affected genes and have SCA. The genetic makeup that causes SCA cannot be developed or acquired through environmental factors, infection, or lifestyle choices later in life.
Sickle Cell Trait vs. Sickle Cell Anemia
Confusion about acquiring the condition later in life often stems from the difference between sickle cell anemia (SCA) and sickle cell trait (SCT). SCA is the full disease, where an individual inherits two copies of the abnormal hemoglobin S gene, resulting in mostly sickle-shaped red blood cells and chronic health issues.
In contrast, SCT occurs when a person inherits only one copy of the abnormal gene and one normal gene. People with SCT have a mix of both normal hemoglobin A and abnormal hemoglobin S. Because they have sufficient normal hemoglobin, they are generally asymptomatic and do not have the disease.
A diagnosis of SCT may occur during adulthood, often during screening for genetic carrier status, military service, or after a child is diagnosed with SCA. This late diagnosis can create the false impression that the condition was newly acquired. While SCT is usually benign, it is a separate, less severe condition than SCA, though extreme conditions like severe dehydration or intense physical exertion can rarely trigger complications in carriers.
Why Symptoms Can Appear Later in Life
Although the genetic condition is present from birth, severe symptoms of sickle cell anemia do not always manifest immediately. In most infants with SCA, symptoms are masked by fetal hemoglobin (HbF), an oxygen-carrying protein produced before birth. HbF is highly effective at preventing the sickling of red blood cells.
The body naturally switches from producing HbF to adult hemoglobin S around five to six months of age, which is typically when the first severe symptoms begin to appear. Until this switch is complete, high levels of HbF offer a protective effect, delaying the onset of painful crises and other complications. For some individuals, a higher proportion of HbF may naturally persist into adulthood, which is associated with a milder form of the disease.
The timing of a severe symptom appearing can also relate to external triggers. Episodes of intense pain, known as vaso-occlusive crises, are often brought on by factors such as dehydration, exposure to high altitude, or physical and emotional stress. These environmental factors may only be encountered later in life, leading a person to experience their first major crisis years after the genetic condition was established.