Figure 1.

HbS fiber formation and red blood cell sickling.

A: Adult hemoglobin (HbA) molecules are heterotetramers composed of two α-and two β-globin subunits and normally exist as solitary molecules inside red blood cells. B: In sickle cell disease (SCD), a substitution in the sixth amino acid of the β-globin subunit causes a structural aberration (HbS) that leads to polymerization of tetramers upon deoxygenation. These tetramers then align to form rod-like fibers that cause the deformation of the red blood cells into the characteristic sickle shape. RBC: red blood cell.