Sickle cell disease
Also known as: sickle cell anemia, SCD
Sickle cell disease is an inherited blood disorder in which a mutation in the beta-globin gene produces abnormal hemoglobin S, causing red blood cells to deform into rigid sickle shapes that block small vessels and break down prematurely.
Sickle cell disease (SCD) is an autosomal recessive disorder of hemoglobin. A single point mutation in the beta-globin gene substitutes valine for glutamic acid at the sixth position, producing hemoglobin S (HbS). When oxygen levels drop, HbS molecules polymerize into stiff rods that distort red blood cells into the characteristic sickle shape.
Sickled cells cause two core problems. They are fragile and destroyed early, producing a chronic hemolytic anemia. And they are rigid and sticky, occluding small blood vessels — the source of painful vaso-occlusive crises, dactylitis in young children, acute chest syndrome, stroke, and priapism. Repeated splenic infarction leads to functional autosplenectomy, leaving patients vulnerable to encapsulated organisms such as Streptococcus pneumoniae and to Salmonella osteomyelitis. Parvovirus B19 infection can trigger a transient aplastic crisis.
Carriers with sickle cell trait (one HbS allele) are generally asymptomatic, and the trait's protection against severe malaria explains the mutation's persistence in populations from malaria-endemic regions. Management concepts include newborn screening, prophylactic penicillin and vaccination in childhood, and hydroxyurea, which raises fetal hemoglobin (HbF) and reduces sickling. Access to care and outcomes in SCD are also a frequently cited example of health disparities.
Sickle cell disease appears across health-professions exams: the USMLE Step 1 tests the mutation, hemolysis, autosplenectomy, and crisis triggers; the CCMA covers it among common diseases medical assistants encounter; and the MCAT references SCD in both molecular genetics and health disparities contexts.
Key takeaways
- Sickle cell disease is autosomal recessive, caused by a glutamic acid-to-valine substitution at position 6 of beta-globin.
- Deoxygenated HbS polymerizes, sickling red cells and causing hemolytic anemia plus vaso-occlusive complications.
- Autosplenectomy raises the risk of infection by encapsulated organisms; parvovirus B19 can cause aplastic crisis.
- Sickle cell trait is usually asymptomatic and confers protection against severe malaria.
- Hydroxyurea helps by increasing fetal hemoglobin, which interferes with HbS polymerization.