Sickle Cell Anaemia - HB3 Gene

HbA is normal adult hemoglobin (α2β2), made of two alpha and two beta globin chains. In sickle cell a mutation in the beta‑globin gene produces hemoglobin S (HbS). When HbA is absent or very low, red cells sickle and cause symptoms. Presence of HbA—naturally in trait or after transfusion—reduces sickling and helps distinguish trait (HbAS) from disease (HbSS).

HbSC refers to a genotype of sickle cell disease in which a person inherits one hemoglobin S (HbS) mutation and one hemoglobin C (HbC) mutation in the beta‑globin (HBB) gene. This combined abnormality causes red cell deformation, hemolytic anemia and vaso‑occlusive symptoms. Clinical severity is often intermediate to milder than HbSS but can still cause pain crises, splenic dysfunction, retinopathy and other complications.

Hb C trait means a person carries one mutated HBB gene that produces hemoglobin C. Carriers are usually asymptomatic or have mild hemolysis and slight anemia. If someone inherits HbC from one parent and HbS (the sickle cell gene) from the other, they have HbSC disease — a type of sickle cell disease causing anemia, painful crises, higher infection risk, and possible organ complications, often milder than HbSS.

Sickle cell anemia is caused by a mutation in the HBB gene, which encodes the beta‑globin subunit of hemoglobin. A single‑base change (glutamic acid→valine at codon 6) creates hemoglobin S (HbS). The condition is inherited in an autosomal recessive pattern: two mutated HBB copies cause sickle cell disease, while one copy typically causes sickle cell trait.

HbA (hemoglobin A) is the predominant adult hemoglobin with two alpha and two beta chains (α2β2), accounting for about 95–97% of adult hemoglobin. HbA2 is a minor adult form with two alpha and two delta chains (α2δ2), typically 2–3.5% of total. HbA2 measurement is clinically useful, for example, to help detect beta‑thalassemia trait when levels are elevated.

The four main types of sickle cell crisis are: vaso-occlusive (pain) crisis — painful ischemic episodes from blocked blood flow; aplastic crisis — sudden severe anemia from halted red cell production (often due to parvovirus B19); splenic sequestration — rapid pooling of blood in the spleen causing acute anemia and hypovolemia; and hemolytic crisis — increased red cell destruction with jaundice and worsening anemia.